- Writing Messages
- Connecting Calls to Catalogs
- Handling Catalog Files
- Semantic Markup
- Localizing Non-Text Resources
- Further References
Internationalization (i18n) is the process of preparing the source code such that the running program can receive and output user-visible information in various human languages, or, more precisely, locales. This is because the same language may be spoken at several places which differ in other details of relevance to the software internationalization (e.g. dialect, calendar, etc). I18n is performed by the programmer. Localization (l10n) is the process of adapting relevant program resources for a particular locale. L10n is performed by the translator, who is usually a native of the locale.
User interface text is the most prominent resource needing l10n. In the source code, there are two general approaches to writing i18n'd user-visible strings, hereafter called messages. In the first approach, the programmer writes symbolic identifier in place of actual message text, and then for each language there is a file, called translation catalog, which contains the actual text linked to the appropriate identifier. For example, in the code:
and in the translation catalog:
In the second approach, the programmer writes actual text inside the source code, in an agreed upon human language, which is usually American English. Then there is a translation catalog for every other language, in which English and translated text of the message are linked. For example, in the code:
and in the translation catalog:
One may observe that the second approach is technically a generalization of the first approach. However, in actual use there are non-negligible differences in the i18n/l10n workflow between these approaches, and therefore they are treated as qualitatively different.
Both approaches – the symbolic-identifier and the actual-text – have some advantages and disadvantages over each other. That is why today both are in use, in software i18n in general. Within free software, though, the actual-text approach is considerably more widespread, as embodied in the GNU Gettext i18n system. Whether this is a historical accident, or due to sensibilities of people writing free software, is left to the interested reader to research.
The KDE Ki18n library, being built atop Gettext, provides the actual-text approach. In the following it is described how to use Ki18n, from the viewpoint of programmers and maintainers. Basic instructions are split into three parts:
- The first part deals with how to write i18n messages in the code. This is the largest and the most important part, since there are many nuances to preparing quality messages for translation. Section: write_i18n.
- The second part describes how to connect particular translation calls with particular translation catalogs, so that the system can know in which catalogs to search for translations. In particular, there are some differences here depending on whether the piece of code is an application or a library. Section: link_cat.
- The third part explains how to extract messages from the code, in order to create the translation template. The template is simply a clean translation catalog (having only English part of each message), which the translator copies over and fills in the translations for a particular language. Then it is shown how and where to install translation catalogs, and how to update existing translation catalogs to newly extracted templates (after some new development has been done, causing new messages to appear, some to be removed, and some modified). Unlike in the GNU Gettext manual, here there are no dependencies or references to a particular software build system; this is left to the maintainer to choose. Section: handle_cat.
There are also two special topics:
- Some programmers like to have more structure and consistency in message texts, and for them Ki18n provides a customizable semantic markup. Section: kuit_markup.
- Sometimes there are program resources other than text that require localization. Ki18n provides a generic, though rudimentary solution for such cases. Section: non_text.
Most messages can be internationalized with simple
i18n* calls, which are described in the section gen_usage. A few messages may require treatment with
ki18n* calls, and when this is needed is described in the section spec_usage. Argument substitution in messages is performed using the familiar Qt syntax
%<number>, but there may be some differences; the section subs_notes gives some notes about placeholders. Finally, guidelines for writing good messages are given in sections good_text and good_ctxt.
General messages are wrapped with
i18n* calls. These calls are immediate, which means that they return the final localized text (including substituted arguments) as a
QString object, that can be passed to UI widgets.
The most frequent message type, a simple text without any arguments, is handled like this:
The message text may contain arbitrary Unicode characters, and the source file must be UTF-8 encoded. Ki18n supports no other character encoding.
If there are some arguments to be substituted into the message,
%<number> placeholders are put into the text at desired positions, and arguments are listed after the string:
Arguments can be of any type for which there exists an overloaded
KLocalizedString::subs method. Up to 9 arguments can be inserted in this fashion, due to the fact that
i18n calls are realized as overloaded templates. If more than 9 arguments are needed, which is extremely rare, a
ki18n* call (described later) must be used.
Sometimes a short message in English is ambiguous to translators, possibly leading to a wrong translations. Ambiguity can be resolved by providing a context string along the text, using the
i18nc call. In it, the first argument is the context, which only the translator will see, and the second argument is the text which the user will see:
Section good_ctxt gives a few pointers on when contexts are needed, and what they should contain.
In messages stating how many of some kind of objects there are, where the number of objects is inserted at run time, it is necessary to differentiate between plural forms of the text. In English there are only two forms, one for number 1 (singular) and another form for any other number (plural). In other languages this might be more complicated (more than two forms), or it might be simpler (same form for all numbers). This is handled properly by using the
i18np plural call:
The plural form is decided by the first integer-valued argument, which is
numImages in this example. In rare cases when there are two or more integer arguments, they should be ordered carefully.
In QML code, due to some limitations, always the first argument will be treated as plural-deciding, so it should be an appropriate number (integer or a round double); otherwise, behavior is undefined.
It is also allowed to omit the plural-deciding placeholder, for example:
If the code context is such that the number is always greater than 1, the plural call must be used nevertheless. This is because in some languages there are different plural forms for different classes of numbers; in particular, the singular form may be used for numbers other than 1 (e.g. those ending in 1).
If a message needs both context and plural forms, this is provided by
In the basic
i18n call (no context, no plural) it is not allowed to put a literal string as the first argument for substitution. In debug mode this will even trigger a static assertion, resulting in compilation error. This serves to prevent misnamed calls: context or plural frequently needs to be added at a later point to a basic call, and at that moment the programmer may forget to update the call name from
Furthermore, an empty string should never be wrapped with a basic
i18n call (no
i18n("")), because in translation catalog the message with empty text has a special meaning, and is not intended for client use. The behavior of
i18n("") is undefined, and there will be some warnings in debug mode.
There is also a complement of
i18nd* calls (
i18ndcp), which are not supposed to be used directly, but as will be explained in the section link_cat.
There are some situations where
i18n* calls are not sufficient, or are not convenient enough. One obvious case is if more than 9 arguments need to be substituted. Another case is if it would be easier to substitute arguments later on, after the line with the i18n call. For cases such as these,
ki18n* calls can be used. These calls are deferred, which means that they do not return the final translated text as
QString, but instead return a
KLocalizedString instance which needs further treatment. Arguments are then substituted one by one using
KLocalizedString::subs methods, and after all arguments have been substituted, the translation is finalized by one of
KLocalizedString::toString methods (which return
QString). For example:
subs methods do not update the
KLocalizedString instance on which they are invoked, but return a copy of it with one argument slot filled. This allows to use
KLocalizedString instances as a templates for constructing final texts, by supplying different arguments.
Another use for deferred calls is when special formatting of arguments is needed, like requesting the field width or number of decimals.
subs methods can take these formatting parameters. In particular, arguments should not be formatted in a custom way, because
subs methods will also take care of proper localization (e.g. use either dot or comma as decimal separator in numbers, etc):
i18n, there are context, plural, and context-plural variants of
toString methods can be used to override the global locale. To override only the language of the locale,
toString can take a list of languages for which to look up translations (ordered by decreasing priority):
The section link_cat describes how to specify the translation domain, a canonical name for the catalog file from which
*i18n* calls will draw translations. But
toString can always be used to override the domain for a given call, by supplying a specific domain:
Relevant here is the set of
ki18nd* calls (
ki18ndcp), which can be used for the same purpose, but which are not intended to be used directly. This will also be covered in the section link_cat.
Translators are provided with the capability to script translations, such that the text changes based on arguments supplied at run time. For the most part, this feature is transparent to the programmer. However, sometimes the programmer may help in this by providing a dynamic context to the message, through
KLocalizedString::inContext methods. Unlike the static context, the dynamic context changes at run time; translators have the means to fetch it and use it to script the translation properly. An example:
Any number of dynamic contexts, with different keys, can be added like this. Normally every message with a dynamic context should also have a static context, like in the previous example, informing the translator of the available dynamic context keys and possible values. Like
inContext does not modify the parent instance, but returns a copy of it.
Messages before creation of Q*Application instance
Fetching the translated messages only works after the Q*Application instance has been created. So any code which will be executed before and which handles text that should be translated has to delay the actual lookup in the catalog (like other locale-dependent actions), in one of two ways: either by using statically initialized character arrays wrapped in
I18N_* macros for extraction, and later making the actual
i18n* calls (see section i18n_noop); or by using
ki18n* calls to create
KLocalizedString instances, which do not perform immediate catalog lookup, and later fetching the actual translation through their toString() methods (see section spec_usage);
One reason for this is that Gettext follows the standard C/POSIX locale settings. By standard on the start of a program all locale categories are fixed to the "C" locale, including the locale for the message catalog category (LC_MESSAGES). To use instead the locales specified by the environment variables, the locale values in the runtime need to be set to an empty string. This is usually done in one go for all locale categories by this call:
From this point on the locale specific environment variables "LANGUAGE", "LC_*" and "LANG" are considered for deciding which locale to use for which category. This includes Gettext when picking the message catalog to use.
The constructor of QCoreApplication (and thus its subclasses) does a call like that. Which means any
i18n* calls done after the creation of the QCoreApplication instance (or of its subclasses) will use a message catalog based on the locale specific environment variables, as one usually expects, But any calls before will use a message catalog for the "C" locale.
Hopefully, most of the time
%<number> placeholders are substituted in the way one would intuitively expect them to be. Nevertheless, some details about substitution are as follows.
Placeholders are substituted in one pass, so there is no need to worry about what will happen if one of the substituted arguments contains a placeholder, and another argument is substituted after it.
All same-numbered placeholders are substituted with the same argument.
Placeholders directly index arguments: they should be numbered from 1 upwards, without gaps in the sequence, until each argument is indexed. Otherwise, error marks will be inserted into message at run time (when the code is compiled in debug mode), and any invalid placeholder will be left unsubstituted. The exception is the plural-deciding argument in plural calls, where it is allowed to drop its placeholder, in either the singular or the plural text.
If none of the arguments supplied to a plural call is integer-valued, an error mark will be inserted into the message at run time (when compiled in debug mode).
Integer arguments will be by default formatted as if they denote an amount, according to locale rules (thousands separation, etc.) But sometimes an integer is a numerical identifier (e.g. port number), and then it should be manually converted into
QString beforehand to avoid treatment as amount:
Writing Good Texts
When writing message texts, sometimes it is tempting to assemble text from pieces such as to have less repetition. However, such shortcuts frequently cannot work for other languages, and are almost always confusing to translators. The first rule of writing good message texts is therefore to keep sentences together and clearly structured (or "no word puzzles"), even at the cost of some repetition.
At its basic, this rule means always to use placeholders for insertion of run time arguments, rather than string concatenation. For example:
This is rather obvious, since it also results in clearer and shorter code. But sometimes placeholders can be overused:
The shorter version here is bad, because the sentence structure of translation may need to change by more than the one word, and also because a less thorough translator may fail to check in which way the short messages "online" and "offline" are used.
If an otherwise long text needs to differ in only small part, then a reasonable solution is to split it at sentence level, but also explain the situation to translators through context:
The third version above is bad because, firstly, the translator may wonder about the trailing space in the first message or simply overlook it, and secondly, there may be some cross-dependency between the translation of the long message and the short messages. In general, insertions of one message into another should always be accompanied by contexts, and composition-significant leading and trailing whitespace should be avoided.
The second basic rule of writing good texts is to expose every user-visible text for translation. One should never make assumptions of the type "this does not need translation" or "it will be same in every language".
One example where programmers sometimes make such assumption are compositions without any letters:
Here, in some languages the arrangement of whitespace will need to differ (e.g. a space may be needed before the colon as well). Letter-free compositions should also normally be equipped with context, because, for example, separation may depend on type of arguments.
Another example of user-visible texts sometimes wrongly omitted from i18n are proper names:
Proper names too may need localization, for example, transliteration when the target language uses a different writing system. This holds for proper names of people, and of anything else.
When it comes to text markup, like the HTML subset supported by Qt rich text processing, opinions are divided on how much of it to expose for translation. One argument goes that markup may be confusing for translators, and that exposing it increases the chance of syntactical errors in translation. This is true as such, but it should be balanced with situations where, either, the translator needs to modify the markup, or, the markup will convey some context to translator. For example, typographical modifiers should always be left in:
because the target language may have different typographical standards (e.g. CJK languages tend to avoid boldface in text body font sizes, as it makes ideographs harder to recognize). Especially if tags are found around internal parts of the message text, it would be ungainly to hide them, e.g. by placeholder insertion. But, values to tag attributes, such as links in
<a href='...'>, tags, should be inserted through placeholders (unless it is expected that translators provide localized values). In this example:
<p> tags could be avoided by splitting this message into one message per sentence, but this should not be done, because paragraphs should be translated as single units of meaning.
Another important point about XML-like text markup, is to try and keep it well-formed in XML sense on the level of standalone message. For example:
Well-formedness is good because the most frequent error in translation in presence of markup is mistyping (or miscopying) a tag. If the original text is well-formed, a translation checker tool can require the same of translation, and signal when that is not so. The previous example of non-well-formedness was perhaps trivial; in practice, non-trivial examples usually break some other rules too (e.g. no word puzzles).
Writing Good Contexts
The message context, given as first argument in
*i18nc calls, is of great help to translators. Unfortunately, to a programmer it is not always clear when a context is needed, or what it should state. So, the very first rule of writing good contexts is to listen to the translators asking for contexts. When taking suggestions from translators, there is no need to worry if the proposed context will be sufficient for "all" languages. It is fine to simply add the information that a translator into particular language requested, and wait for translators into other languages to maybe request some other context information as well.
Having said this, some apriori guidelines on contexts can be followed.
Since in English the form of an adjective does not change based on the gender of the noun it modifies, properly translating messages which are single standalone adjectives will be impossible in many languages without a context. So, in general, every message which is a standalone adjective should have context:
Lists of related items typically benefit from having the same context, since they should all be translated in the same style:
When there are placeholders in the text for which it is not clear, from the text alone, what kind of argument they represent, this should be explained in the context:
It is frequently suggested to state in the context the grammar category of the message text, such as "this is a verb" or "this is a noun". Since the grammar category of the translation does not have to be the same as that of the original, this kind of context provides circumstantial information at best (see the section uimark_ctxt for what translators may use it to draw some real information about), and is worthless at worst. Also, due to relative absence of declension in English grammar, different programmers may have different opinion on the grammar category: the menu title "View", is it a verb or a noun?
User Interface Markers
In the same way there exists a HIG (Human Interface Guidelines) document for the programmers to follow, translators should establish HIG-like convention for their language concerning the forms of UI text. Therefore, for a proper translation, the translator will need too know not only what does the message mean, but also where it figures in the UI. E.g. is the message a button label, a menu title, a tooltip, etc.
To this end a convention has been developed among KDE translators, which programmers can use to succinctly describe UI usage of messages. In this convention, the context string starts with an UI marker of the form
@<major>:<minor>, and may be followed by any other usual context information, separated with a single space:
The major and minor component of the UI marker are not arbitrary, but are drawn from the following table. For each component, the superscript states the Ki18n release when the component was introduced.
|@action5.0||Labels of clickable widgets which cause an action to be performed.|
|:button5.0||Push buttons in windows and dialogs.|
|:inmenu5.0||Menu entries that perform an action (as opposed e.g. to being checked).|
|@title5.0||Text that is the title of a major UI element or a widget container.|
|:window5.0||Title of a window or a (dockable) view/pane.|
|:group5.0||Title to a group of widgets, like a group of checkboxes or radio buttons.|
|:column5.0||Column name in a table header, e.g. in a table view widget.|
|:row5.0||Row name in a table.|
|@option5.0||Labels of option selection widgets, which can be enable/disabled or selected between.|
|:check5.0||Checkbox label, also a checkable menu entry.|
|:radio5.0||Radio button label.|
|@label5.0||Various widget labels which are not covered by any of @action, @title, or @option.|
|:listbox5.0||Label to a list box or combo box.|
|:textbox5.0||Label to a text box or text edit field.|
|:chooser5.0||Label to any special-purpose chooser widget, like color chooser, font chooser, etc.|
|@item5.0||Strings that are items from a range of possibilities or properties of a given type.|
|:inmenu5.0||Item presented in menu (e.g. sort ordering, encoding name, etc).|
|:inlistbox5.0||Item presented in a list or combo box.|
|:intable5.0||Item presented in a table cell.|
|:inrange5.0||End range labels, e.g. on sliders.|
|:intext5.0||Words and short phrases which are inserted into a larger piece of text.|
|:valuesuffix5.46||Suffix appended to a value, including any spacing (e.g. in a spinbox).|
|@info5.0||Any transient information for the user.|
|:tooltip5.0||Expanded formulation of the widget's label, usually appearing automatically when the pointer hovers over the widget.|
|:whatsthis5.0||Longer description of a widget's purpose and behavior, usually manually called up by the user.|
|:placeholder5.46||A hint what input is expected in an input field, shown in the place of the input where there is none yet.|
|:status5.0||A piece of text displayed in application's status view (e.g in the status bar).|
|:progress5.0||Text describing the current step or state of an operation, possibly periodically updating.|
|:usagetip5.0||A tip that comes up to inform the user about a certain possibility in a given context, e.g. a "tip of the day" on application startup.|
Deprecated synonym: :tipoftheday.
|:credit5.0||Contributor names and their contributions, e.g. in the about dialog.|
|:shell5.0||A note, warning or error sent to application's text output stream (stdout, stderr) rather than shown in the UI.|
If none of the minor components apply to a given message, a major component can be used standalone. For example, this would happen with a library-provided list of items without any immediate UI context (e.g. language or country names), where the appropriate UI marker would be just
One way to extend the context after the UI marker, which is simple for the programmer yet can be very helpful for translators, is simply to add the text of the (technically or logically) parent widget. For example, if the action "New" is in the menu "File", then:
Or, if the item "Left" is found in the list box with the label "Vertical alignment":
Adding Contexts in Non-C++ files
When Qt Designer is used to build the user interface, the -tr option of
uic should be used to pass UI strings through Ki18n's
tr2i18n function (see also link_ui). This function is the equivalent of
i18nc, based on whether the second argument is null or not. If a string in the
.ui file has the attribute
comment=, its value will be automatically used as the context argument. (In Qt Designer, this is the "disambiguation" property of a string.) Alternatively, strings can be passed to Ki18n's
tr2xi18n function (see kuit_markup).
In KXmlGui (
.rc) and KConfigXT (
.kcfg) files, contexts can be added through
context= attributes to
Disambiguation Context vs. Extracted Comment
The GNU Gettext system actually defines two types of context for translators. The type discussed so far, the first argument of
*i18nc* calls, is called disambiguation context. The other type of context is extracted comment. In Ki18n, this other type of context is written as a code comment, in the line just preceding the message text, and starting with i18n: keyword:
There are two main differences between disambiguation contexts and extracted comments.
The first difference is that extracted comments do not separate messages in the translation catalog. For example, such two messages equipped with extracted comments:
will show up in the translation catalog as a single message with aggregate comments:
The same two messages equipped with disambiguation contexts:
will show up in the translation catalog as two messages:
The second difference is that a change in extracted comment does not invalidate the existing translation, i.e. it will not force translators to revisit the message and revise the translation (see the section handle_update for details on how this invalidation happens with disambiguation contexts). Thus, for example, if there is a "message freeze" before the next release of a piece of software, during which programmers must not modify messages so that translators can thoroughly complete their work, it is allowed to modify the extracted comment, but it is not allowed to modify the disambiguation context.
The Gettext manual suggests to use extracted comments as the default way of providing context for translators, and to use disambiguation contexts only when message separation is necessary. However, we (the people in the KDE community) suggest the opposite – to use disambiguation context by default, and extracted comments only in special cases. This because there are two dangers associated with extracted comments: one is that programmer may fail to properly judge when two messages should be made separate for translation (and having to judge that all the time is a burden in the first place), and the other is that when the context change is such that translators really should revisit the message, they would not get any automatic signal about that. The message freeze advantage of extracted comments has not been observed to be very important.
One special case when to an use extracted comment is when the context is a multi-sentence text explaining the purpose of the message, and the context is unlikely to change (in its meaning). Another case is when the context lists a fixed set of possible translations ("meta-messages", by which translators influence some aspect of text formatting), which may expand in the future, as new possibilities are introduced at request of translators into new languages.
Sometimes it is convenient only to mark a message for extraction (into the catalog template, as described in handle_extract), and to place the actual
i18n call somewhere else. A typical case of this are global static initializers, where only POD types can be safely used. For this purpose
I18N*_NOOP macros are provided.
I18N_NOOP macro is the counterpart to
*i18n calls, and it is used like this:
I18NC_NOOP macro is the counterpart to
There are also two deprecated macros:
I18N_NOOP2_NOSTRIP is simply the old name of
I18N_NOOP2 takes the context argument but discards it, which means that the context must be repeated verbatim in the corresponding
I18N*_NOOP macros make it harder to follow i18n in the code, and should be avoided when possible.
Connecting Calls to Catalogs
i18n call must look for translations in exactly one translation catalog for a given language of translation. For this purpose, a group of catalogs which have the same source text but translations into different languages, is identified by a unique canonical name, called the domain. Therefore, every
i18n call must be connected to a domain. This connection is established differently for applications and libraries, though the difference is only for convenience: if desired, the more verbose library method can be used for application code as well.
Connecting to Catalogs in Application Code
*i18n* calls in an application can be connected to a single domain by calling the static
KLocalizedString::setApplicationDomain method with the domain as the argument:
This call must be made in the code before any
i18n call takes place, and right after creating the instance of
QCoreApplication or one of its subclasses.
ki18n calls can still be made before, but the respective
KLocalizedString::toString() has to be delayed to after that.
This is all there is to connecting calls and catalogs application's C++ source files. However, there may also be some non-code files that need connecting, and how to do this is some typical non-code files is described in the section link_noncode.
Connecting to Catalogs in Library Code
i18n calls in libraries must be strictly connected to library's own translation domain no matter how the library is used, and this should be fully transparent to the library's client. In particular, if the client does not use Ki18n for internationalization of its own messages, library translation must still work as expected. Therefore, in library code, the call-domain connection is established in this way:
The definition of
TRANSLATION_DOMAIN triggers the domain-specialization macro in
klocalizedstring.h. It routes all
*i18n* calls to their
*i18nd* counterparts, which take the domain as their first argument. The
i18n call from this example will thus expand into:
It is possible to use
*i18nd* calls explicitly, but there should be no need for that. If there are any messages that should draw translations from a special domain, it is better style-wise to use the
TRANSLATION_DOMAIN can be put into a private header file of the library, so that it does not have to be repeated at multiple locations. If there are some
i18n calls in a public header file, definition of
TRANSLATION_DOMAIN would propagate into and affect the application client code that uses Ki18n too. This is prevented by adding the following lines somewhere after the last
i18n call in the public header:
This will undefine all expansions of
*i18nd*, leaving the client's environment clean. If instead the public header contains only
I18N_NOOP* macros, defining
TRANSLATION_DOMAIN is unnecessary in the first place, since actual
i18n calls happen somewhere else.
Connecting to Catalogs in Non-Code Files
Both KDE applications and libraries can include some non-code files which contain messages that need to be connected to a translation domain. This can be the same domain where messages from C++ code are found, or another domain, whatever seems more appropriate. In principle, each type of non-code file requires its own connection mechanism, and here it is explained how this works for typical types of non-code files found in KDE sources.
It is assumed in the following that all messages in the non-code file are connected to the single domain. In other words, the connection is specified on the file level rather than on the message level.
Qt Designer (.ui) files
First, to have UI strings from
.ui file passed through Ki18n,
uic is run with
-tr tr2i18n. This will replace all native Qt
tr calls with Ki18n's
tr2i18n calls in the resulting header file. Then, the generated header file needs to be post-processed to fix empty messages and include
klocalizedstring.h. At this point, the
TRANSLATION_DOMAIN can be defined just like in static C++ files.
If CMake is used as the build system, a macro that performs all of the above is provided (
ki18n_wrap_ui). Otherwise, one could use a shell snippet such as this:
If strings contain KUIT markup (section kuit_markup),
tr2i18n in the lines above should be replaced with
KXmlGui (.rc) files
.rc files are interpreted at runtime, the translation domain connection is established simply by adding the
translationDomain attribute to the top element:
.rc file belongs to application rather than library source, it is not necessary to set
translationDomain. If not set, translations will be looked up in the domain set with
KLocalizedString::setApplicationDomain call in the code.
If strings contain KUIT markup (section kuit_markup), additionally the attribute
translationMarkup="true" should be set.
KConfigXT (.kcfg) files
Instructions for building the configuration code from a
.kcfg file are contained in the
.kcfgc file of the same base name;
kconfig_compiler is invoked with both files as arguments. Then, the domain connection is established simply by adding the
TranslationDomain fields in the
.kcfgc file, to select Ki18n as the translation system and the appropriate translation domain:
.kcfg file is part of an application rather than a library, the
TranslationDomain field can be omitted in order to have messages looked up in the domain set by
KLocalizedString::setApplicationDomain call in the code.
If strings contain KUIT markup (section kuit_markup), additionaly the field
TranslationMarkup=true should be set.
Handling Catalog Files
For translators to start working, one or more translation catalog files should be prepared, based on the
i18n calls in the source code. The procedure to do this is called extraction of messages. Extraction produces empty catalog files, called templates. These files are in the PO format, and have
.pot extension. Section handle_extract explains how to perform extraction.
Once templates are ready, a translators make copies of them, with
.po extension, and start filling them out with translations into respective languages. When translation is done, the translated catalog is committed into the source code repository. The build system is set up to install translated catalogs. Section handle_install provides necessary steps for this.
After some development has passed, the source repository will contain many translated catalogs which are out of date with respect to latest catalog templates. Of course, translators do not have to start translating from scratch, but there are specialized tools to carry over as much of existing translation as possible, so that only new and modified texts need to be considered. Section handle_update shows how this is done.
A usual application or a library has one translation catalog, but there can be more if there is higher modularity of the source. The following subsections refer to a single catalog wherever the extension to case with multiple catalogs is obvious, and mention multiple catalogs only where necessary.
The primary tool for collecting texts from
i18n calls and writing out the catalog template is
xgettext, from the official Gettext tools package.
xgettext supports many programming languages and sublanguage environments, among which naturally C++ and Ki18n specifically.
The extraction process from source code files is thus simple:
xgettext runs with appropriate options over all files, and it writes out the catalog template. For the moment masking the complete list of options as
xgettext can be run for example like this at the top of Fooapp source to create the catalog template
Or, a list of source files that should be extracted from can be assembled separately and fed to
One may want to assemble the list of source files by hand or semi-automatically in order to prioritize the order of translation (messages from most important files appearing first in the catalog), to exclude some portions of the source tree from extraction, and so on.
$EXTOPTS that cover everything from Ki18n, and some generalities, should look like this:
--c++ --kde options tell
xgettext that source files are C++ with Ki18n.
--from-code=UTF-8 specifies the encoding of source files to be UTF-8, which must be so for Ki18n.
-c i18n states that comments for extraction start with given keyword (
// i18n: ...). The series of
-k options informs
xgettext of all possible translation call names and which of their arguments to extract. Finally, options
--msgid-bugs-address automatically write the corresponding information into the catalog at proper place. If there are semantic markup calls in the code (section kuit_markup), the following
-k options should be added as well:
xgettext unfortunately cannot be directly used to extract messages from the usual XML files appearing in Qt and KDE sources – Designer (
.ui), KXmlGui (
.rc) and KConfigXT (
.kcfg) files. Therefore the
kdesdk package provides the
extractrc script, which extracts XML messages as dummy
i18n calls into a dummy C++ file, This file can then be included into the list of files for
xgettext run. The usual invocation of
If the catalog being extracted is an application catalog, i.e. given as
KLocalizedString::setApplicationDomain in the code, it should contain two meta-messages for translation credits which will be shown by
KAboutApplicationDialog. These messages are also written as dummy
i18n calls, usually into the same dummy C++ file with XML messages, with the following context and text:
The extraction command sequence can be written down as a small script, to be run periodically by the maintainer, or it can be integrated into the build system.
For the code residing in the official KDE repositories a special form of the extraction script is mandated. This enables automatic overnight template extraction and feeding into dedicated translation section of KDE repositories. Details can be found at KDE Techbase.
Placing and Installing Catalogs
For an application or a library which uses a single catalog, the usual organization of catalogs in the source tree is this:
Here translated catalog files are named by their language codes (
cc...). In case of multiple catalogs, one directory per catalog can be created under the
An alternative organization is to have one directory per language, and name catalog files by the translation domain. In multiple catalog situation this would look like:
Catalog templates are fully derived files, and therefore some maintainers do not like to keep them inside the repository. In that case, at least the tarball should contain the templates (i.e. they should be generated at packaging time), so that translators have somewhere to get them from. Another possibility is to upload the template to a translation hub (such as Transifex), which translators can use to upload translated catalogs back, and usually for some other features as well (assignment, review, etc).
If the code resides in an official KDE repository, neither templates nor translated catalogs are kept inside the source tree. Instead, translated catalogs are fetched from an appropriate place when tarball is made, using a script provided for that purpose. Details can be found at KDE Techbase.
No matter how the catalog files are named and organized inside the distribution tarball, they must be installed in exactly one way. The base name of the installed catalog must be their translation domain, and if the package is installed into
$PREFIX, the installed directory tree must look like as follows:
Given that these directories are shared with other packages in the same prefix, by Gettext convention translation domains must be unique (like package names are).
MO files are the compiled version of PO files, and they are produced using Gettext's
Placing and Installing Scripting Modules
Scripting modules can be placed like this in the source tree:
or in the per-language directory variant:
The installation location for scripting modules is like that for catalogs, only using
LC_SCRIPTS/ directory instead of
When a translator inquires about adding a scripting module, or sends one in, the maintainer should check with the translator if perhaps the functions provided by the module are more widely applicable. If that is the case, they should rather become part of Ki18n's own scripting module, because then they will be accessible to all Ki18n-based translations in the given language.
When new catalog template is extracted after some development has been done, existing translation should be updated against it. This is called merging with template, and it is performed by Gettext's
msgmerge command. There are some merging options that can be examined here, but generally the best invocation of
msgmerge is this:
--update --backup=none mean to update the catalog in place, and not to make a backup file. Option
--previous puts some additional information into every modified message, that translation editing tools can use to show to the translator all changes in the text. This command should be run once for every existing translation catalog.
One thing to keep in mind is that a change in the context string of a message in the code (i.e. the first argument to
*i18nc* calls), including adding or removing one, will also register as a modified message in the merged catalog. This will require that translators revisit it, which is exactly as intended: if the context has changed, especially if it was added, some changes in translation may be needed. However, this means that when a "message freeze" is declared so that translators can complete updating translations without disruption, contexts fall under same rules as text.
There are a few possibilities for who and when should perform merging. For example, the maintainer can write a script that at the same time extracts the template and merges all catalogs, and run it periodically, committing updated catalogs to the repository for translators to pick up. This could even be integrated into the build system. Some maintainers do not like committing automatic changes, and instead expect translators to run the extraction-merging script for the language they maintain, update the translation, and commit only that updated catalog. This solution is cleaner with respect to repository history, but it may burden translators.
When operating in an official KDE repository, maintainers do not have to deal with merging at all. The server-side automation which automatically extracts templates and provides them to translators, also performs merging. So the maintainer is left only to pick up whatever are the latest catalogs when making a tarball.
When composing user-interface text, some programmers ponder about the typographical conventions to use in certain contexts. For example, when a file name is inserted into the text, some typographical solutions that can be used are:
For the Qt widgets that have rich text capability, exposed as subset of HTML tags, additional solutions include:
The problem here is not so much to decide on one solution, as it is to follow it consistently through time and between contributors. One may also want to use two solutions, one in places where only plain text is allowed, and another where rich text is available. Wouldn't it then be easier to write everywhere:
and have it automatically resolve into plain or rich text according to the UI context, using formatting patterns defined at one place? This approach is called semantic markup, because the author marks parts of the text according to what they represent.
Ki18n implements such a semantic markup, called KUIT (KDE User Interface Text). It is accessed through the series of
kxi18n* calls, which are the KUIT-aware counterparts of
ki18n* calls. Ordinary and KUIT-aware calls can be freely mixed within a given body of code.
KUIT defines a number of semantic tags that are frequently of use, as listed in the section kuit_tags. But, KUIT also allows the programmer to define custom tags, as well as to change visual formatting patterns for predefined tags. This capability should lessen one important issue of semantic markups: when the author is forced to choose between several tags none of which exactly fits the desired meaning; with KUIT, the author can simply define a custom tag in that case.
Tags are defined and redefined per translation domain, so that changes will not affect markup resolution in any other domain. Changes are performed through the
KuitSetup object associated with the domain, as returned by the
Kuit::setupForDomain method. A tag is defined (or redefined) by defining (or redefining) its formatting patterns, with one call to
KuitSetup::setTagPattern for each desired combination of tag name, attribute names, and visual format. Here is an example of defining the tag
<player>, which has an optional attribute
color=, on the domain
The first two
setTagPattern calls set up resolution of
<player> without attributes, into plain and rich text. The third call sets up resolution for
<player color="...">, but only into rich text; since a plain text pattern is not defined for this tag-attribute combination, it will fall back to basic
<player> plain text pattern. A fallback is always defined, the elementary fallback being a no-op, where tag is simply removed. Formatting patterns must be wrapped for translation too, since translators may need to tweak them; ordinary (not markup-aware)
ki18nc calls must be used here, since patterns themselves are not KUIT markup. The
%1 placeholder in the pattern will be replaced by the text wrapped with the tag, and
%2 and upwards with attribute values, in the order of appearance in attribute names list.
If a simple substitution pattern is insufficient for formatting, additionally a formatting function of type
Kuit::TagFormatter can be given. The result of this function is substituted into the pattern; alternatively an empty pattern can be given (as
KLocalizedString()), in which case the result is used directly, no substitution is performed. The formatting function also receives the current element path, so that the resolution can depend on the markup tree context if needed. Anything in the function that may need translator input should be appropriately exposed through
In the section kuit_tags it is stated that every KUIT tag is classified either as a phrase tag or as a structuring tag, and explained what that means for processing. A newly defined tag is by default a phrase tag; method
KuitSetup::setTagClass can be used to change its class:
In a library, changes to the KUIT setup may need to be private, applicable only in library's own domain, but they may also need to be public, applicable in clients' domains. For changes that should be public, the library should define a public function which takes the domain as the argument and performs all the changes on that domain:
The client code should then call this function in its initialization.
Selecting Visual Format
The target visual format for any given
xi18n call can be selected in two ways.
The primary way is by UI markers in the message context, which were described in the section uimark_ctxt. Every
@<major>:<minor> marker combination has a default target visual format assigned, as follows:
|UI Marker||Visual Format|
|@info, @info:tooltip, @info:whatsthis, @info:usagetip||rich|
|@info:status, @info:progress, @info:credit||plain|
Target visual formats associated with UI markers can be changed using the
The second way to select the visual format is by using a
kxi18n* call, and passing the format type to the
This will override any format implied by the UI marker if present.
If a library is making modifications in visual format association with UI markers, and these changes should be available to clients, the same approach as in the section kuit_def_tags should be used.
*i18n* calls it was advised to keep each message text well-formed by itself with respect to Qt rich text markup, for
*xi18n* calls well-formedness is mandatory. This means that markup-significant characters in plain-looking text need to be escaped, using standard XML entities:
The exception is the ampersand (&) character, which in XML denotes the start of an entity, but in Qt denotes the accelerator marker. Therefore it is necessary to escape ampersand as
& only when it is in position which would result in valid entity syntax:
The example for necessary escaping above is rather artificial, because in practice it is unlikely for ampersand to appear in entity-like position while not actually starting an entity.
To assure the validity of markup when arguments are inserted into
xi18n-wrapped text, all markup-significant characters in string arguments are automatically escaped. For example, this works as expected:
But, how then to compose a text where the arguments too should contain some KUIT markup? This is done by using non-finalization
kxi18n* call to translate the argument text, and passing the returned
KLocalizedString object directly as argument:
If the argument and the text have different visual formats implied by their UI markers, the outermost format overrides inner formats.
All KUIT tags belong to one of the two classes:
- phrase tags, which describe parts of sentences or whole sentences inserted into running text;
- structuring tags, which split text into paragraph-level blocks.
A text without any structuring tags is considered equivalent of one paragraph or sub-paragraph sentence or phrase. If at least one structuring tag appears in the text, then the text is considered multi-paragraph, and no content may appear outside of structuring tags. For example:
The current set of predefined tags is presented below. For each tag the following information is stated: the tag name (in superscript: Ki18n release of first appearance), available attributes (in superscript: * if mandatory, Ki18n release of first appearance), admissible subtags, and description.
Emphasized word or phrase in the text. For strong emphasis, attribute
Subtags: all phrase tags
A list item.
List of items. List is considered an element of the paragraph, so
Subtags: all phrase tags, <list>
One paragraph of text.
Subtags: all phrase tags
The subtitle of the text. Must come after
Subtags: all phrase tags
The title of the text. Must be the first tag in the text if present.
The criteria for adding new tags to the predefined set, particularly new phrase tags, are not very strict. If the tag is clearly useful to a class of applications, of which more than one are known to use Ki18n, it is reasonable to add it here. Adding synonymous tag names is also fine, where one finds that the original name is not sufficiently discoverable, or that it is too verbose for the given frequency of use.
KUIT defines a fixed set of XML entities, which means that unlike tags, entities cannot be added to, nor their character expansions changed. The standard XML entities are:
|'||single quote (|
|"||double quote (|
" are really needed only within attribute values (and even there they can be avoided by choosing the opposite quote sign for quoting the attribute value).
Additional entities are (Ki18n release where they were introduced given in superscript):
| 5.0||non-breaking space (|
The reason for not allowing custom entities can be demonstrated by the following example. An application programmer may think of defining the entity
&appname;, which would be used everywhere in text in place of the actual application name. In that way, seemingly, it would be easy to play with various names or spellings without disrupting translations. The problem, however, is that in many languages the structure of the sentence depends on the grammar properties of the particular name (e.g. its grammar gender or number), and conversely, the name may need modification according to sentence structure (e.g. by grammar case). Thus, custom entities are not allowed because they are misused too easily. If something really needs to be inserted verbatim into text, argument substitution is always at hand.
Localizing Non-Text Resources
It sometimes happens that a non-textual application resource needs localization. The most frequent example are images that contain some text, like splash screens. Ki18n also provides a rudimentary facility for this situation, the
KLocalizedString::localizedFilePath static method. When called with a resource file path as the argument, this method will check what are the active languages, and look if there exists a localized version of the resource at path
<original-parent-dir>/l10n/<language>/<original-basename>. For example, if the active language is
aa, and a candidate image for localization is installed as:
then a call to
will check if there exist the file
and return that path if it does, or else the original path.
Some KDE libraries will call
KLocalizedString::localizedFilePath on their own behind the scene, for resources that may need localization but whose paths are not directly manipulated in application sources. An example here are icons handled through
KIcon class, which are referred to in the code only by the icon name.
For details about the format of translation catalogs (PO) and various Gettext tools, the first stop is the Gettext manual.