source: trunk/doc/src/internationalization/i18n.qdoc@ 561

/****************************************************************************
**
** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
** All rights reserved.
** Contact: Nokia Corporation (qt-info@nokia.com)
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** Commercial Usage
** Licensees holding valid Qt Commercial licenses may use this file in
** accordance with the Qt Commercial License Agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and Nokia.
**
** GNU Lesser General Public License Usage
** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 2.1 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL included in the
** packaging of this file.  Please review the following information to
** ensure the GNU Lesser General Public License version 2.1 requirements
** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
**
** In addition, as a special exception, Nokia gives you certain additional
** rights.  These rights are described in the Nokia Qt LGPL Exception
** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
**
** GNU General Public License Usage
** Alternatively, this file may be used under the terms of the GNU
** General Public License version 3.0 as published by the Free Software
** Foundation and appearing in the file LICENSE.GPL included in the
** packaging of this file.  Please review the following information to
** ensure the GNU General Public License version 3.0 requirements will be
** met: http://www.gnu.org/copyleft/gpl.html.
**
** If you have questions regarding the use of this file, please contact
** Nokia at qt-info@nokia.com.
** $QT_END_LICENSE$
**
****************************************************************************/

/*!
    \group i18n
    \title Qt Classes for Internationalization

    See \l{Internationalization with Qt} for information on how to use these classes
    in your applications.
*/

/*!
    \page internationalization.html
    \title Internationalization with Qt
    \brief Information about Qt's support for internationalization and multiple languages.
    \nextpage Writing Source Code for Translation

    \keyword internationalization
    \keyword i18n

    The internationalization of an application is the process of making
    the application usable by people in countries other than one's own.

    \tableofcontents

    \section1 Relevant Qt Classes and APIs

    These classes support internationalizing of Qt applications.

    \annotatedlist i18n

    \section1 Languages and Writing Systems

    In some cases internationalization is simple, for example, making a US
    application accessible to Australian or British users may require
    little more than a few spelling corrections. But to make a US
    application usable by Japanese users, or a Korean application usable
    by German users, will require that the software operate not only in
    different languages, but use different input techniques, character
    encodings and presentation conventions.

    Qt tries to make internationalization as painless as possible for
    developers. All input widgets and text drawing methods in Qt offer
    built-in support for all supported languages. The built-in font engine
    is capable of correctly and attractively rendering text that contains
    characters from a variety of different writing systems at the same
    time.

    Qt supports most languages in use today, in particular:
    \list
    \o All East Asian languages (Chinese, Japanese and Korean)
    \o All Western languages (using Latin script)
    \o Arabic
    \o Cyrillic languages (Russian, Ukrainian, etc.)
    \o Greek
    \o Hebrew
    \o Thai and Lao
    \o All scripts in Unicode 5.1 that do not require special processing
    \endlist

    On Windows, Unix/X11 with FontConfig (client side font support)
    and Qt for Embedded Linux the following languages are also supported:
    \list
    \o Bengali
    \o Devanagari
    \o Dhivehi (Thaana)
    \o Gujarati
    \o Gurmukhi
    \o Kannada
    \o Khmer
    \o Malayalam
    \o Myanmar
    \o Syriac
    \o Tamil
    \o Telugu
    \o Tibetan
    \o N'Ko
    \endlist

    Many of these writing systems exhibit special features:

    \list

    \o \bold{Special line breaking behavior.} Some of the Asian languages are
    written without spaces between words. Line breaking can occur either
    after every character (with exceptions) as in Chinese, Japanese and
    Korean, or after logical word boundaries as in Thai.

    \o \bold{Bidirectional writing.} Arabic and Hebrew are written from right to
    left, except for numbers and embedded English text which is written
    left to right. The exact behavior is defined in the
    \l{http://www.unicode.org/unicode/reports/tr9/}{Unicode Technical Annex #9}.

    \o \bold{Non-spacing or diacritical marks (accents or umlauts in European
    languages).} Some languages such as Vietnamese make extensive use of
    these marks and some characters can have more than one mark at the
    same time to clarify pronunciation.

    \o \bold{Ligatures.} In special contexts, some pairs of characters get
    replaced by a combined glyph forming a ligature. Common examples are
    the fl and fi ligatures used in typesetting US and European books.

    \endlist

    Qt tries to take care of all the special features listed above. You
    usually don't have to worry about these features so long as you use
    Qt's input widgets (e.g. QLineEdit, QTextEdit, and derived classes)
    and Qt's display widgets (e.g. QLabel).

    Support for these writing systems is transparent to the
    programmer and completely encapsulated in \l{rich text
    processing}{Qt's text engine}. This means that you don't need to
    have any knowledge about the writing system used in a particular
    language, except for the following small points:

    \list

    \o QPainter::drawText(int x, int y, const QString &str) will always
    draw the string with its left edge at the position specified with
    the x, y parameters. This will usually give you left aligned strings.
    Arabic and Hebrew application strings are usually right
    aligned, so for these languages use the version of drawText() that
    takes a QRect since this will align in accordance with the language.

    \o When you write your own text input controls, use QTextLayout.
    In some languages (e.g. Arabic or languages from the Indian
    subcontinent), the width and shape of a glyph changes depending on the
    surrounding characters, which QTextLayout takes into account.
    Writing input controls usually requires a certain knowledge of the
    scripts it is going to be used in. Usually the easiest way is to
    subclass QLineEdit or QTextEdit.

    \endlist

    The following sections give some information on the status of the
    internationalization (i18n) support in Qt. See also the \l{Qt
    Linguist manual}.

    \section1 Step by Step

    Writing cross-platform international software with Qt is a gentle,
    incremental process. Your software can become internationalized in
    the following stages:

    \section2 Use QString for All User-Visible Text

    Since QString uses the Unicode 5.1 encoding internally, every
    language in the world can be processed transparently using
    familiar text processing operations. Also, since all Qt functions
    that present text to the user take a QString as a parameter,
    there is no \c{char *} to QString conversion overhead.

    Strings that are in "programmer space" (such as QObject names
    and file format texts) need not use QString; the traditional
    \c{char *} or the QByteArray class will suffice.

    You're unlikely to notice that you are using Unicode;
    QString, and QChar are just like easier versions of the crude
    \c{const char *} and char from traditional C.

    \section2 Use tr() for All Literal Text

    Wherever your program uses "quoted text" for text that will
    be presented to the user, ensure that it is processed by the \l
    QCoreApplication::translate() function. Essentially all that is necessary
    to achieve this is to use QObject::tr(). For example, assuming the
    \c LoginWidget is a subclass of QWidget:

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 0

    This accounts for 99% of the user-visible strings you're likely to
    write.

    If the quoted text is not in a member function of a
    QObject subclass, use either the tr() function of an
    appropriate class, or the QCoreApplication::translate() function
    directly:

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 1

    If you need to have translatable text completely
    outside a function, there are two macros to help: QT_TR_NOOP()
    and QT_TRANSLATE_NOOP(). They merely mark the text for
    extraction by the \c lupdate utility described below.
    The macros expand to just the text (without the context).

    Example of QT_TR_NOOP():

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 2

    Example of QT_TRANSLATE_NOOP():

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 3

    If you disable the \c{const char *} to QString automatic
    conversion by compiling your software with the macro \c
    QT_NO_CAST_FROM_ASCII defined, you'll be very likely to catch any
    strings you are missing. See QString::fromLatin1() for more
    information. Disabling the conversion can make programming a bit
    cumbersome.

    If your source language uses characters outside Latin1, you
    might find QObject::trUtf8() more convenient than
    QObject::tr(), as tr() depends on the
    QTextCodec::codecForTr(), which makes it more fragile than
    QObject::trUtf8().

    \section2 Use QKeySequence() for Accelerator Values

    Accelerator values such as Ctrl+Q or Alt+F need to be translated
    too. If you hardcode Qt::CTRL + Qt::Key_Q for "quit" in your
    application, translators won't be able to override it. The
    correct idiom is

    \snippet examples/mainwindows/application/mainwindow.cpp 20

    \section2 Use QString::arg() for Dynamic Text

    The QString::arg() functions offer a simple means for substituting
    arguments:
    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 4

    In some languages the order of arguments may need to change, and this
    can easily be achieved by changing the order of the % arguments. For
    example:

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 5

    produces the correct output in English and Norwegian:
    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 6

    \section2 Produce Translations

    Once you are using tr() throughout an application, you can start
    producing translations of the user-visible text in your program.

    The \l{Qt Linguist manual} provides further information about
    Qt's translation tools, \e{Qt Linguist}, \c lupdate and \c
    lrelease.

    Translation of a Qt application is a three-step process:

    \list 1

    \o Run \c lupdate to extract translatable text from the C++
    source code of the Qt application, resulting in a message file
    for translators (a TS file). The utility recognizes the tr()
    construct and the \c{QT_TR*_NOOP()} macros described above and
    produces TS files (usually one per language).

    \o Provide translations for the source texts in the TS file, using
    \e{Qt Linguist}. Since TS files are in XML format, you can also
    edit them by hand.

    \o Run \c lrelease to obtain a light-weight message file (a QM
    file) from the TS file, suitable only for end use. Think of the TS
    files as "source files", and QM files as "object files". The
    translator edits the TS files, but the users of your application
    only need the QM files. Both kinds of files are platform and
    locale independent.

    \endlist

    Typically, you will repeat these steps for every release of your
    application. The \c lupdate utility does its best to reuse the
    translations from previous releases.

    Before you run \c lupdate, you should prepare a project file. Here's
    an example project file (\c .pro file):

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 7

    When you run \c lupdate or \c lrelease, you must give the name of the
    project file as a command-line argument.

    In this example, four exotic languages are supported: Danish,
    Finnish, Norwegian and Swedish. If you use \l{qmake}, you usually
    don't need an extra project file for \c lupdate; your \c qmake
    project file will work fine once you add the \c TRANSLATIONS
    entry.

    In your application, you must \l QTranslator::load() the translation
    files appropriate for the user's language, and install them using \l
    QCoreApplication::installTranslator().

    \c linguist, \c lupdate and \c lrelease are installed in the \c bin
    subdirectory of the base directory Qt is installed into. Click Help|Manual
    in \e{Qt Linguist} to access the user's manual; it contains a tutorial
    to get you started.

    \target qt-itself
    Qt itself contains over 400 strings that will also need to be
    translated into the languages that you are targeting. You will find
    translation files for French, German and Simplified Chinese in
    \c{$QTDIR/translations}, as well as a template for translating to
    other languages. (This directory also contains some additional
    unsupported translations which may be useful.)

    Typically, your application's \c main() function will look like
    this:

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 8

    Note the use of QLibraryInfo::location() to locate the Qt translations.
    Developers should request the path to the translations at run-time by
    passing QLibraryInfo::TranslationsPath to this function instead of
    using the \c QTDIR environment variable in their applications.

    \section2 Support for Encodings

    The QTextCodec class and the facilities in QTextStream make it easy to
    support many input and output encodings for your users' data. When an
    application starts, the locale of the machine will determine the 8-bit
    encoding used when dealing with 8-bit data: such as for font
    selection, text display, 8-bit text I/O, and character input.

    The application may occasionally require encodings other than the
    default local 8-bit encoding. For example, an application in a
    Cyrillic KOI8-R locale (the de-facto standard locale in Russia) might
    need to output Cyrillic in the ISO 8859-5 encoding. Code for this
    would be:

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 9

    For converting Unicode to local 8-bit encodings, a shortcut is
    available: the QString::toLocal8Bit() function returns such 8-bit
    data. Another useful shortcut is QString::toUtf8(), which returns
    text in the 8-bit UTF-8 encoding: this perfectly preserves
    Unicode information while looking like plain ASCII if the text is
    wholly ASCII.

    For converting the other way, there are the QString::fromUtf8() and
    QString::fromLocal8Bit() convenience functions, or the general code,
    demonstrated by this conversion from ISO 8859-5 Cyrillic to Unicode
    conversion:

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 10

    Ideally Unicode I/O should be used as this maximizes the portability
    of documents between users around the world, but in reality it is
    useful to support all the appropriate encodings that your users will
    need to process existing documents. In general, Unicode (UTF-16 or
    UTF-8) is best for information transferred between arbitrary people,
    while within a language or national group, a local standard is often
    more appropriate. The most important encoding to support is the one
    returned by QTextCodec::codecForLocale(), as this is the one the user
    is most likely to need for communicating with other people and
    applications (this is the codec used by local8Bit()).

    Qt supports most of the more frequently used encodings natively. For a
    complete list of supported encodings see the \l QTextCodec
    documentation.

    In some cases and for less frequently used encodings it may be
    necessary to write your own QTextCodec subclass. Depending on the
    urgency, it may be useful to contact Qt's technical support team or
    ask on the \c qt-interest mailing list to see if someone else is
    already working on supporting the encoding.

    \keyword localization

    \section2 Localize

    Localization is the process of adapting to local conventions, for
    example presenting dates and times using the locally preferred
    formats. Such localizations can be accomplished using appropriate tr()
    strings.

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 11

    In the example, for the US we would leave the translation of
    "AMPM" as it is and thereby use the 12-hour clock branch; but in
    Europe we would translate it as something else and this will make
    the code use the 24-hour clock branch.

    For localized numbers use the QLocale class.

    Localizing images is not recommended. Choose clear icons that are
    appropriate for all localities, rather than relying on local puns or
    stretched metaphors. The exception is for images of left and right
    pointing arrows which may need to be reversed for Arabic and Hebrew
    locales.

    \section1 Dynamic Translation

    Some applications, such as Qt Linguist, must be able to support changes
    to the user's language settings while they are still running. To make
    widgets aware of changes to the installed QTranslators, reimplement the 
    widget's \l{QWidget::changeEvent()}{changeEvent()} function to check whether 
    the event is a \l{QEvent::LanguageChange}{LanguageChange} event, and update
    the text displayed by widgets using the \l{QObject::tr()}{tr()} function
    in the usual way. For example:

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 12

    All other change events should be passed on by calling the default
    implementation of the function.

    The list of installed translators might change in reaction to a 
    \l{QEvent::LocaleChange}{LocaleChange} event, or the application might
    provide a user interface that allows the user to change the current
    application language.

    The default event handler for QWidget subclasses responds to the
    QEvent::LanguageChange event, and will call this function when necessary.

    \l{QEvent::LanguageChange}{LanguageChange} events are posted when a new
    translation is installed using the QCoreApplication::installTranslator()
    function. Additionally, other application components can also force
    widgets to update themselves by posting LanguageChange events to them.


    \section1 Translating Non-Qt Classes

    It is sometimes necessary to provide internationalization support for
    strings used in classes that do not inherit QObject or use the Q_OBJECT
    macro to enable translation features. Since Qt translates strings at
    run-time based on the class they are associated with and \c lupdate
    looks for translatable strings in the source code, non-Qt classes must
    use mechanisms that also provide this information.

    One way to do this is to add translation support to a non-Qt class
    using the Q_DECLARE_TR_FUNCTIONS() macro; for example:

    \snippet doc/src/snippets/i18n-non-qt-class/myclass.h 0
    \dots
    \snippet doc/src/snippets/i18n-non-qt-class/myclass.h 1

    This provides the class with \l{QObject::}{tr()} functions that can
    be used to translate strings associated with the class, and makes it
    possible for \c lupdate to find translatable strings in the source
    code.

    Alternatively, the QCoreApplication::translate() function can be called
    with a specific context, and this will be recognized by \c lupdate and
    Qt Linguist.

    \section1 System Support

    Some of the operating systems and windowing systems that Qt runs on
    only have limited support for Unicode. The level of support available
    in the underlying system has some influence on the support that Qt can
    provide on those platforms, although in general Qt applications need
    not be too concerned with platform-specific limitations.

    \section2 Unix/X11

    \list
    \o  Locale-oriented fonts and input methods. Qt hides these and
        provides Unicode input and output.
    \o  Filesystem conventions such as
        \l{http://www.ietf.org/rfc/rfc2279.txt}{UTF-8}
        are under development in some Unix variants. All Qt file
        functions allow Unicode, but convert filenames to the local
        8-bit encoding, as this is the Unix convention (see
        QFile::setEncodingFunction() to explore alternative
        encodings).
    \o  File I/O defaults to the local 8-bit encoding,
        with Unicode options in QTextStream.
    \o  Many Unix distributions contain only partial support for some locales.
        For example, if you have a \c /usr/share/locale/ja_JP.EUC directory,
        this does not necessarily mean you can display Japanese text; you also
        need JIS encoded fonts (or Unicode fonts), and the
        \c /usr/share/locale/ja_JP.EUC directory needs to be complete. For
        best results, use complete locales from your system vendor.
    \endlist

    \section2 Windows

    \list
    \o  Qt provides full Unicode support, including input methods, fonts,
        clipboard, drag-and-drop and file names.
    \o  File I/O defaults to Latin1, with Unicode options in QTextStream.
        Note that some Windows programs do not understand big-endian
        Unicode text files even though that is the order prescribed by
        the Unicode Standard in the absence of higher-level protocols.
    \o  Unlike programs written with MFC or plain winlib, Qt programs
        are portable between Windows 98 and Windows NT.
        \e {You do not need different binaries to support Unicode.}
    \endlist

    \section2 Mac OS X

    For details on Mac-specific translation, refer to the Qt/Mac Specific Issues
    document \l{Qt for Mac OS X - Specific Issues#Translating the Application Menu and Native Dialogs}{here}.
*/

/*!
    \page i18n-source-translation.html
    \title Writing Source Code for Translation
    \ingroup i18n
    \previouspage Internationalization with Qt
    \contentspage Internationalization with Qt
    \nextpage Translation Rules for Plurals
    \brief How to write source code in a way that makes it possible for user-visible text to be translated.

    \tableofcontents

    \section1 The Basics

    Developers use the \l{QObject::}{tr()} function to obtain translated text
    for their classes, typically for display purposes. This function is also
    used to indicate which text strings in an application are translatable.

    Qt indexes each translatable string by the \e{translation context} it is
    associated with; this is generally the name of the QObject subclass it is
    used in.

    Translation contexts are defined for new QObject-based classes by the use
    of the Q_OBJECT macro in each new class definition.

    When tr() is called, it looks up the translatable string using a QTranslator
    object. For translation to work, one or more of these must have been
    installed on the application object in the way described in the
    \l{#Enabling Translation}{Enabling Translation} section below.

    \section1 Defining a Translation Context

    The translation context for QObject and each QObject subclass is the
    class name itself. Developers subclassing QObject must use the
    Q_OBJECT macro in their class definition to override the translation
    context. This macro sets the context to the name of the subclass.

    For example, the following class definition includes the Q_OBJECT macro,
    implementing a new tr() that uses the \c MainWindow context:

    \snippet mainwindows/sdi/mainwindow.h class definition with macro
    \dots

    If Q_OBJECT is not used in a class definition, the context will be
    inherited from the base class. For example, since all QObject-based
    classes in Qt provide a context, a new QWidget subclass defined without
    a Q_OBJECT macro will use the \c QWidget context if its tr() function
    is invoked.

    \section1 Using tr() to Obtain a Translation

    The following example shows how a translation is obtained for the
    class shown in the previous section:

    \snippet mainwindows/sdi/mainwindow.cpp implicit tr context
    \dots

    Here, the translation context is \c MainWindow because it is the
    \c MainWindow::tr() function that is invoked. The text returned
    by the tr() function is a translation of "&File" obtained from
    the \c MainWindow context.

    When Qt's translation tool, \l lupdate, is used to process a set of source
    files, the text wrapped in tr() calls is stored in a section of the translation
    file that corresponds to its translation context.

    In some situations, it is useful to give a translation context explicitly
    by fully qualifying the call to tr(); for example:

    \snippet doc/src/snippets/code/src_corelib_kernel_qobject.cpp explicit tr context

    This call obtains the translated text for "Page up" from the \c QScrollBar
    context. Developers can also use the QCoreApplication::translate() function
    to obtain a translation for a particular translation context.

    \section1 Translator Comments

    Developers can include information about each translatable string to
    help translators with the translation process. These are extracted
    when \l lupdate is used to process the source files. The recommended
    way to add comments is to annotate the tr() calls in your code with
    comments of the form:

    \tt{//: ...}

    or

    \tt{\begincomment: ... \endcomment}

    Examples:

    \snippet doc/src/snippets/code/src_corelib_kernel_qobject.cpp 40

    In these examples, the comments will be associated with the strings
    passed to tr() in the context of each call.

    \section1 Adding Meta-Data to Strings

    Additional data can be attached to each translatable message. These are
    extracted when \l lupdate is used to process the source files. The
    recommended way to add meta-data is to annotate the tr() calls in your code
    with comments of the form:

    \tt{//= <id>}

    This can be used to give the message a unique identifier to support tools
    which need it.

    An alternative way to attach meta-data is to use the following syntax:

    \tt{//~ <field name> <field contents>}

    This can be used to attach meta-data to the message. The field name should
    consist of a domain prefix (possibly the conventional file extension of the
    file format the field is inspired by), a hyphen and the actual field name
    in underscore-delimited notation. For storage in TS files, the field name
    together with the prefix "extra-" will form an XML element name. The field
    contents will be XML-escaped, but otherwise appear verbatim as the
    element's contents. Any number of unique fields can be added to each
    message.

    Example:

    \snippet doc/src/snippets/code/src_corelib_kernel_qobject.cpp meta data

    Meta-data appearing right in front of a magic TRANSLATOR comment applies to
    the whole TS file.

    \section1 Disambiguation

    If the same translatable string is used in different roles within the same
    translation context, an additional identifying string may be passed in
    the call to \l{QObject::}{tr()}. This optional disambiguation argument
    is used to distinguish between otherwise identical strings.

    Example:

    \snippet doc/src/snippets/code/src_corelib_kernel_qobject.cpp 17
    \dots

    In Qt 4.4 and earlier, this disambiguation parameter was the preferred
    way to specify comments to translators.

    \section1 Character Encodings

    You can set the encoding for the source text by calling QTextCodec::setCodecForTr().
    By default, the source text is assumed to be in Latin-1 encoding.

    \section1 Handling Plurals

    Some translatable strings contain placeholders for integer values and need
    to be translated differently depending on the values in use.

    To help with this problem, developers pass an additional integer argument
    to the \l{QObject::}{tr()} function, and typically use a special notation
    for plurals in each translatable string.

    If this argument is equal or greater than zero, all occurrences of
    \c %n in the resulting string are replaced with a decimal representation
    of the value supplied. In addition, the translation used will adapt to the
    value according to the rules for each language.

    Example:

    \snippet doc/src/snippets/code/src_corelib_kernel_qobject.cpp 18

    The table below shows what string is returned depending on the
    active translation:

    \table
    \header \o      \o{3,1} Active Translation
    \header \o \a n \o No Translation        \o French                                 \o English
    \row    \o 0    \o "0 message(s) saved"  \o "0 message sauvegard\unicode{0xE9}"    \o "0 message\bold{s} saved"
    \row    \o 1    \o "1 message(s) saved"  \o "1 message sauvegard\unicode{0xE9}"    \o "1 message saved"
    \row    \o 2    \o "2 message(s) saved"  \o "2 message\bold{s} sauvegard\unicode{0xE9}\bold{s}"  \o "2 message\bold{s} saved"
    \row    \o 37   \o "37 message(s) saved" \o "37 message\bold{s} sauvegard\unicode{0xE9}\bold{s}" \o "37 message\bold{s} saved"
    \endtable

    This idiom is more flexible than the traditional approach; e.g.,

    \snippet doc/src/snippets/code/src_corelib_kernel_qobject.cpp 19

    because it also works with target languages that have several
    plural forms (e.g., Irish has a special "dual" form that should
    be used when \c n is 2), and it handles the \e n == 0 case
    correctly for languages such as French that require the singular.
    See the \l{Qt Linguist Manual} for details.

    Instead of \c %n, you can use \c %Ln to produce a localized
    representation of \a n. The conversion uses the default locale,
    set using QLocale::setDefault(). (If no default locale was
    specified, the "C" locale is used.)

    A summary of the rules used to translate strings containing plurals can be
    found in the \l{Translation Rules for Plurals} document.

    \section1 Enabling Translation

    Typically, your application's \c main() function will look like
    this:

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 8

    Note the use of QLibraryInfo::location() to locate the Qt translations.
    Developers should request the path to the translations at run-time by
    passing QLibraryInfo::TranslationsPath to this function instead of
    using the \c QTDIR environment variable in their applications.

    \section1 Further Reading

    \l{Qt Linguist Manual}, \l{Hello tr Example}, \l{Translation Rules for Plurals}
*/

/*!
    \page i18n-plural-rules.html
    \title Translation Rules for Plurals
    \ingroup i18n
    \previouspage Writing Source Code for Translation
    \contentspage Internationalization with Qt
    \brief A summary of the translation rules for plurals produced by Qt's i18n tools.

    The table below shows the specific rules that are produced by Qt Linguist
    and \c lrelease for a selection of languages. Cells marked \e otherwise
    indicate the form used when none of the other rules are appropriate for a
    specific language.

    \table 80%
    \header \o Language \o Rule 1 \o Rule 2 \o Rule 3
    \row \o English \o \c{n == 1} 
        \o \e{otherwise} \o N/A
    \row \o French \o \c{n < 2} 
        \o \e{otherwise} \o N/A
    \row \o Czech \o \c{n % 100 == 1} 
        \o \c{n % 100 >= 2 && n % 100 <= 4} 
        \o \e{otherwise}
    \row \o Irish \o \c{n == 1} 
        \o \c{n == 2} \o \e{otherwise}
    \row \o Latvian \o \c{n % 10 == 1&& n % 100 != 11} 
        \o \c{n != 0} \o \e{otherwise}
    \row \o Lithuanian \o \c{n % 10 == 1&& n % 100 != 11} 
        \o \c{n % 100 != 12 && n % 10 == 2} 
        \o \e{otherwise}
    \row \o Macedonian \o \c{n % 10 == 1} 
        \o \c{n % 10 == 2} \o \e{otherwise}
    \row \o Polish \o \c{n == 1} 
        \o \c{n % 10 >= 2 && n % 10 <= 4
            && (n % 100 < 10 || n % 100 > 20)} 
        \o \e{otherwise}
    \row \o Romanian \o \c{n == 1} 
        \o \c{n == 0|| (n % 100 >= 1 && n % 100 <= 20)} 
        \o \e{otherwise}
    \row \o Russian \o \c{n % 10 == 1&& n % 100 != 11} 
        \o \c{n % 10 >= 2 && n % 10 <= 4
            && (n % 100 < 10 || n % 100 > 20)} 
        \o \e{otherwise}
    \row \o Slovak \o \c{n == 1} \o \c{n >= 2 && n <= 4} 
        \o \e{otherwise}
    \row \o Japanese \o \e{otherwise} \o N/A \o N/A
    \endtable

    The rules themselves are not documented and are internal to Qt Linguist and \c lrelease.
*/