// Copyright (C) 2023 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

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

    \ingroup explanations-accessibility
    \keyword internationalization
    \keyword i18n

    The \e internationalization and \e localization of an application are the
    processes of adapting the application to different languages, regional
    differences, and technical requirements of a target market.

    \list
        \li \l {Writing Source Code for Translation}{Internationalization}
            means designing an application so that it can be adapted to various
            languages and regions without engineering changes.
        \li \l {Localizing Applications}{Localization} means adapting
            internationalized applications for a specific region or language by
            adding locale-specific components (such as date, time, and number
            formats) and translating text.
    \endlist

    The need for internationalization ranges from spelling changes to enabling
    the application to operate in different languages and to use different input
    techniques, character encoding, and presentation conventions.

    All input controls and text drawing methods in Qt offer built-in support for
    all supported languages. The built-in font engine correctly renders text that
    contains characters from a variety of different writing systems at the same
    time.

    \table
    \header
        \li For more information about
        \li See
    \row
        \li Internationalizing source code
        \li \l{Writing Source Code for Translation}
    \row
        \li Configuring and deploying translations, as well as using existing
            Qt module translations
        \li \l {Localizing Applications}
    \row
        \li Using the Qt translation tools
        \li \l {Qt Linguist Manual}
    \endtable

    The following video shows how to internationalize and localize a simple
    example application:

    \youtube xNIz78IPBu0

    \section1 Qt Classes for Internationalization

    The following classes support internationalizing of Qt applications.

    \annotatedlist i18n

    See \l{Writing Source Code for Translation} for more information about how
    to use the classes in applications.

    \section1 Languages and Writing Systems

    Qt supports most \l {QLocale::Language}{languages} in use today.

    Input controls, such as the Qt Quick TextInput type and QLineEdit, QTextEdit,
    and derived classes, as well as display controls, such as the \l Text type
    and QLabel class handle the following special features of the different
    writing systems:

    \list

    \li Line breaks

        Some of the Asian languages are
        written without spaces between words. Line breaking can occur either
        after any character (with exceptions) as in Chinese, Japanese and
        Korean, or after logical word boundaries as in Thai.

    \li 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{https://www.unicode.org/unicode/reports/tr9/}{Unicode Technical Annex #9}.

    \li Non-spacing or diacritical marks, such as 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.

    \li Ligatures

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

    \endlist

    \l{Rich Text Processing}{Qt's text engine} supports different
    \l {QLocale::Script}{writing systems} that work on all platforms
    if the fonts for rendering them are installed.

    You do not need to know about the writing system used in a particular
    language, unless you want to write your own text input controls. In some
    languages, such as Arabic or languages from the
    Indian subcontinent, the width and shape of a glyph changes depending on the
    surrounding characters. To take this into account in C++ code, use QTextLayout.
    Writing input controls also requires some knowledge of the scripts they are
    going to be used in. Usually, the easiest way is to subclass QLineEdit or
    QTextEdit.

    \section1 Encoding

    Encoding is relevant both for application source files and the text files
    that the application reads or writes.

    \section2 Encoding Source Code

    QML documents are always encoded in UTF-8 format. Since Qt 6, 8-bit
    \l{http://www.ietf.org/rfc/rfc2279.txt}{UTF-8} is the predominant encoding
    also in Qt C++.

    The \c lupdate tool extracts UI strings from your application.
    It expects all source code to be encoded in UTF-8 by default.

    However, some editors, such as Visual Studio, use a different
    encoding by default. One way to avoid encoding issues is to limit any
    source code to ASCII, and use escape sequences for translatable strings
    with other characters, for example:

    \code
    label->setText(tr("F\374r \310lise"));
    \endcode

    QString::toUtf8() returns the text in UTF-8 encoding, which preserves
    \l{Unicode in Qt}{Unicode} information while looking like plain ASCII if
    the text is wholly ASCII. To convert Unicode to local 8-bit encoding, use
    QString::toLocal8Bit(). On Unix systems, this is equivalent to \c toUtf8().
    On Windows, the system's current code page is used.

    For converting from UTF-8 and local 8-bit encoding to QString, use the
    QString::fromUtf8() and QString::fromLocal8Bit() convenience functions.

    \section2 Encoding Text Input/Output

    Use QTextStream::setEncoding() to set common encoding for text streams.

    If you need some other legacy encoding, use the QTextCodec class from the
    Qt5Compat module.

    When an application starts, the locale of the machine determines the 8-bit
    encoding used for external 8-bit data. QTextCodec::codecForLocale()
    returns a codec that you can use to convert between this locale encoding
    and Unicode.

    The application may occasionally require encoding 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 snippets/code/doc_src_i18n.cpp 9

    The following code demonstrates the conversion from ISO 8859-5 Cyrillic to
    Unicode:

    \snippet snippets/code/doc_src_i18n.cpp 10

    For a complete list of supported encodings see the \l QTextCodec
    documentation.

    \section1 Operating and Windowing Systems

    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
    \li Qt hides locale-oriented fonts and input methods and
        provides Unicode input and output.
    \li Most Unix variants use filesystem conventions such as UTF-8
        by default. All Qt file functions allow Unicode, but convert
        filenames to the local 8-bit encoding, as this is the Unix convention.
    \li  File I/O defaults to the local 8-bit encoding,
        with Unicode options in QTextStream.
    \li  Some older Unix distributions contain only partial support for some locales.
        For example, even if you have a \c /usr/share/locale/ja_JP.EUC directory,
        you cannot display Japanese text unless you install Japanese fonts and
        the directory is complete. For best results, use complete locales from
        your system vendor.
    \endlist

    \section2 Linux

    \list
    \li  Qt provides full Unicode support, including input methods, fonts,
        clipboard, and drag-and-drop.
    \li The file system is encoded in UTF-8 on all modern Linux distributions.
        File I/O defaults to UTF-8.
    \endlist

    \section2 Windows

    \list
    \li  Qt provides full Unicode support, including input methods, fonts,
        clipboard, drag-and-drop, and file names.
    \li  File I/O defaults to Latin1, with Unicode options in QTextStream.
        However, 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.
    \endlist

    \section1 Related Topics
*/

/*!
    \page i18n-source-translation.html
    \title Writing Source Code for Translation
    \ingroup internationalization
    \previouspage Internationalization with Qt
    \nextpage Localizing Applications
    \brief Writing source code that enables the localization of applications.

    Write QML and Qt C++ source code in a way that enables you to localize
    applications:

    \list
        \li \l {Mark Strings for Translation}
        \li \l {Use Parameters Instead of Concatenating Strings}
        \li \l {Handle Plural Forms}
        \li \l {Use Regional Number Settings}
        \li \l {Internationalize Date, Time, and Currency}
        \li \l {Mark Translatable Data Text Strings}
        \li \l {Add Comments for Translators}
        \li \l {Disambiguate Identical Text}
        \li \l {Make Keyboard Shortcuts Translatable}
        \li \l {Use Locale to Extend Localization Features}
        \li \l {Enable Translation}
        \li \l {Prepare for Dynamic Language Changes}
    \endlist

    When developing C++ applications, see also
    \l {Additional Considerations for C++ Code}.

    \section1 Mark Strings for Translation

    Most of the text that must be translated in an application consists of
    either single words or short phrases. These typically appear as window
    titles, menu items, tooltips, and labels to buttons, check boxes, and
    radio buttons.

    Qt minimizes the performance cost of using translations by
    translating the phrases for each window as they are created. In most
    applications, the main window is created just once. Dialogs are often
    created once and then shown and hidden as required. Once the initial
    translation has taken place, there is no further runtime overhead for
    the translated windows. Only those windows that are created,
    destroyed and subsequently created will have a translation
    performance cost.

    You can create applications that switch language at runtime, but it
    requires an effort and comes with a runtime performance cost.

    Use translation functions to mark user-visible UI text for
    translation in QML and C++ code. Qt indexes each translatable string by
    the \e{translation context} it is associated with.
    The same phrase may occur in more than one context without conflict.
    If a phrase occurs more than once in a particular context, it is translated
    only once and the translation is applied to every occurrence within the
    context.

    \section2 QML: Use qsTr()

    In QML, you can use the following functions to mark user-visible strings for
    translation in .qml files:

    \list
        \li \l [QML] {Qt::}{qsTr()}
        \li \l [QML] {Qt::}{qsTranslate()}
        \li \l [QML] {Qt::}{qsTrId()}
    \endlist

    Usually, you use the \c qsTr() function:

    \code
    Text {
        id: txt1
        text: qsTr("Back")
    }
    \endcode

    This code makes \e Back a key entry in the translation source (TS) files. At
    runtime, the translation system looks up the keyword \e Back and then
    gets the corresponding translation value for the current system locale.
    The result is returned to the \c text property and the UI shows the
    appropriate translation of \e Back for the current locale. If no translation
    is found, \c qsTr() returns the original string.


    The translation context can be set for a given file with:

    \code
    pragma Translator: ChosenContext
    \endcode

    or

    \code
    pragma Translator: "Chosen::Context"
    \endcode

    The context set via \c qsTranslate() takes precedent over the context set via \c pragma \c Translator.
    In QML, by default, the translation context is the file name.

    \section2 C++: Use tr()

    In C++, use the \l{QObject::}{tr()} function to mark text as translatable
    and to display translated text. The translation context is the name of the
    QObject subclass the string is used in. To define translation context for new
    QObject-based classes, use the Q_OBJECT macro in each new class definition.

    When \c tr() is called, it looks up the translatable string using a
    \l QTranslator object, which you must install on the application object,
    as described in \l{Enable Translation}.

    For example, assuming the \c LoginWidget is a subclass of QWidget:

    \snippet snippets/code/doc_src_i18n.cpp 0

    This accounts for 99% of the user-visible strings you're likely to
    write. For information about marking string literals translatable,
    see \l {Mark Translatable Data Text Strings}.

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

    \snippet snippets/code/doc_src_i18n.cpp 1

    \note If you disable the \c{const char *} to QString automatic conversion by
    compiling your application with the macro \c QT_NO_CAST_FROM_ASCII defined,
    you will most likely catch any strings you are missing. See
    QString::fromUtf8() and QString::fromLatin1() for more information.

    \section1 Use Parameters Instead of Concatenating Strings

    Different languages arrange words differently in phrases, clauses, and
    sentences, so do not create strings by concatenating words and data. Instead,
    use \c % to insert parameters into strings.

    For example, in the string \c {After processing file %1, file %2 is next in
    line}, \c{%1} and \c{%2} are numbered parameters. At runtime, \c{%1} and
    \c{%2} are replaced with the first and second file names, respectively. The
    same numbered parameters must appear in the translation, but not necessarily
    in the same order. A German translation of the string might reverse the
    phrases. For example, \c{Datei %2 wird bearbeitet, wenn Datei %1 fertig ist}.
    Both numbered parameters appear in the translation, but in the reverse order.

    \section2 QML: Use .arg()

    The following QML snippet has a string with two number parameters
    \c %1 and \c %2. These parameters are inserted with the \c .arg() functions.

    \code
    Text {
        text: qsTr("File %1 of %2").arg(counter).arg(total)
    }
    \endcode

    \c %1 refers to the first parameter and \c %2 refers to the second parameter,
    so this code produces output like: \e {File 2 of 3}.

    \section2 C++: Use QString::arg()

    In C++, use the QString::arg() functions to substitute parameters:

    \snippet snippets/code/doc_src_i18n.cpp 4

    This code produces output like: \e {5 of 10 files copied. Copying:
    somefile.txt}.

    \section1 Handle Plural Forms

    You can pass an additional integer parameter (\e n) to the translation
    functions and use a special notation for plural forms (\c %n) in each
    translatable string.

    Depending on the value of \e n, the translation function returns a different
    translation, with the correct grammatical number for the target language.
    Also, any occurrence of \c %n is replaced with \e n's value.

    For example, the English and French translations of the string
    \c {%n message(s) saved} require different plural forms.

    \table
    \header
        \li \e n
        \li No Translation
        \li French
        \li English
    \row
        \li 0
        \li "0 message(s) saved"
        \li "0 message sauvegard\unicode{0xE9}"
        \li "0 message\b{s} saved"
    \row
        \li 1
        \li "1 message(s) saved"
        \li "1 message sauvegard\unicode{0xE9}"
        \li "1 message saved"
    \row
        \li 2
        \li "2 message(s) saved"
        \li "2 message\b{s} sauvegard\unicode{0xE9}\b{s}"
        \li "2 message\b{s} saved"
    \row
        \li 37
        \li "37 message(s) saved"
        \li "37 message\b{s} sauvegard\unicode{0xE9}\b{s}"
        \li "37 message\b{s} saved"
    \endtable

    This idiom also works with target languages that have several plural forms,
    such as a \e dual form. In addition, the idiom handles the \e n \c {== 0}
    case correctly for languages such as French that require the singular.

    For a summary of the rules that \QL and \c lrelease use to translate strings
    that contain plural forms, see \l{Translation Rules for Plural Forms}.

    To handle plural forms in the native language, load a TS file for this
    language, too. Use the \c lupdate tool \c -pluralonly command line option,
    to create TS files that contain only entries with plural forms.

    Alternatively, you can use the \c lconvert tool's \c -pluralonly command
    line option to remove all non-plural forms from an existing TS file.

    \section2 QML Example

    The following QML code snippet replaces \c %n with the value of \c total:

    \code
    Text {
        text: qsTr("%n message(s) saved").arg(total)
    }
    \endcode

    \section2 C++ Example

    The following C++ code snippet replaces \c %n with the value that the
    \c {count()} function returns:

    \snippet snippets/code/src_corelib_kernel_qobject.cpp 18

    \section1 Use Regional Number Settings

    If you include the \c %L modifier when you specify a parameter, the number is
    localized according to the current regional settings. The conversion uses the
    default locale if you set it or the system-wide locale, otherwise.

    \section2 QML: Use %L

    For example, in the following QML snippet, \c %L1 formats the first
    parameter according to the number formatting conventions of the currently
    selected locale (geographical region):

    \code
    Text {
        text: qsTr("%L1").arg(total)
    }
    \endcode

    If \c total is the number \e {4321.56}, with English regional settings
    (locale) the output is \e {4,321.56}, whereas with German regional settings
    it is \e {4.321,56}.

    \section2 C++: Use %Ln

    In C++, you can use \c %Ln to produce a localized representation of \c n.
    Use QLocale::setDefault() to set the default locale.

    \keyword localization
    \section1 Internationalize Date, Time, and Currency

    Present date, time, and currency using the locally preferred formats.

    \section2 QML: Use QtQml Functions

    QML does not have special in-string modifiers for formatting dates and times.
    Instead, you need to query the current locale (geographical region) and use
    the methods of \l {QtQml::}{Date} to format the string.

    \c Qt.locale() returns a \l {QtQml::}{Locale} object which contains
    information about the locale. In particular, the \l {QtQml::Locale::name}
    {Locale.name} property contains the language and country of the current
    locale. You can use the value as is or parse it to determine the appropriate
    content for the current locale.

    The following snippet gets the current date and time with \c Date(), then
    converts that to a string for the current locale. Then it inserts the date
    string into the \c %1 parameter for the appropriate translation.

    \code
    Text {
        text: qsTr("Date %1").arg(Date().toLocaleString(Qt.locale()))
    }
    \endcode

    To localize currency numbers, use the \l {QtQml::}{Number} type. It has
    similar functions as the \c Date type for converting numbers into localized
    currency strings.

    \section2 C++: Use QLocale Class

    In C++, use QLocale::timeFormat() or QLocale::toString(QTime) or
    \c toString(QDate):

    \code
    QLabel *label = new QLabel(this);
    label->setText(tr("Date %1").arg(QLocale().toString(QDate::currentDate()));
    \endcode

    \section1 Mark Translatable Data Text Strings

    Use \c _NoOp functions (in QML) and \c _NOOP macros (in C++) to mark
    translatable string literals for extraction by the \c lupdate tool.

    \section2 QML: Use _NoOp Functions

    In QML, use the following functions to mark translatable string literals:

    \list
        \li \l [QML] {Qt::}{qsTrNoOp()}
        \li \l [QML] {Qt::}{qsTranslateNoOp()}
        \li \l [QML] {Qt::}{qsTrIdNoOp()}
    \endlist

    If the user changes the system language without a reboot, depending on the
    system, the strings in arrays and list models and other data structures might
    not be refreshed automatically. To force the texts to be refreshed when they
    are displayed in the UI, you need to declare the strings with the
    \c {qsTrNoOp()} function. Then, when you populate the objects for display,
    you need to explicitly retrieve the translation for each text.

    For example:

    \code
    ListModel {
        id: myListModel

        ListElement {
            //: Capital city of Finland
            name: qsTrNoOp("Helsinki")
        }
    }

    ...

    Text {
        text: qsTr(myListModel.get(0).name)
        // Get the translation of the name property in element 0
    }
    \endcode

    \section2 C++: Use _NOOP Macros

    For translatable text completely outside a function, use the \l QT_TR_NOOP()
    and \l QT_TRANSLATE_NOOP() macros that expand to just the text without the
    context.

    An example of \c QT_TR_NOOP():

    \snippet snippets/code/doc_src_i18n.cpp 2

    An example of \c QT_TRANSLATE_NOOP():

    \snippet snippets/code/doc_src_i18n.cpp 3

    \section1 Add Comments for Translators

    You can add comments in the source code before a string you mark as
    translatable to clarify its purpose. The comments are included in the
    TS files that you deliver to the translator.

    \note The TS files are XML files with the source texts and a place for the
    translated text. The updated TS files are converted into binary translation
    files and included as part of the final application.

    \section2 QML: Use //: and //~

    In the following code snippet, the text on the \c {//:} line is the main comment
    for the translator.

    The text on the \c{//~} line is optional extra information. The first word of
    the text is used as an additional identifier in the XML element in the TS file
    so make sure the first word is not part of the sentence. For example, the
    comment \e {Context Not related to back-stepping} is converted to
    \c {<extra-Context>Not related to back-stepping} in the TS file.

    \code
    Text {
        id: txt1;
        // This UI string is only used here
        //: The back of the object, not the front
        //~ Context Not related to back-stepping
        text: qsTr("Back");
    }
    \endcode

    \section2 C++: Use Comment Characters

    To add comments in C++, annotate the \c tr() calls in your code with
    comments of the form \c {//:} or by marking the beginning and end of
    the comment.

    In the following examples, the comments are associated with the strings
    passed to \c tr() in the context of each call:

    \snippet snippets/code/src_corelib_kernel_qobject.cpp 40

    To add optional comments, use:

    \code
    //~ <field name> <field contents>
    \endcode

    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 \c extra- will form an XML
    element name. The field contents will be XML-escaped, but otherwise appear
    verbatim as the element's contents. You can add any number of unique fields
    to each message.

    Example:

    \snippet snippets/code/src_corelib_kernel_qobject.cpp meta data

    In C++, you use an equals sign to add a unique identifier:

    \code
    //= <id>
    \endcode

    You can use the keyword \e TRANSLATOR for translator comments.
    Metadata appearing right in front of the TRANSLATOR keyword applies to
    the whole TS file.

    \section1 Disambiguate Identical Text

    The translation system consolidates the UI text strings into unique items to
    avoid having to translate the same text multiple times. However, a text
    might look identical to another text but have a different meaning. For
    example, in English, \e back means both a step backward and the part of an
    object opposite to the front. You need to tell the translation system about
    these two separate meanings, so the translator can create two separate
    translations.

    \section2 QML: Add a Disambiguator to qsTr()

    In QML, add a disambiguating string as the second parameter of the \c qsTr()
    function.

    In the following code snippet, the ID \c {not front} differentiates this
    \e Back text from the backstepping \e Back text:

    \code
    Text {
        id: txt1
        // This UI string is used only here
        //: The back of the object, not the front
        //~ Context Not related to back-stepping
        text: qsTr("Back", "not front")
    }
    \endcode

    \section2 C++: Add a Disambiguator to tr()

    In C++, pass a disambiguating string in the call to \l{QObject::}{tr()}.

    In the following code snippet, the ID \c recipient differentiates the name of
    the recipient from that of the sender:

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

    \section1 Make Keyboard Shortcuts Translatable

    In its most common form, a keyboard shortcut describes a combination of keys
    that you press to perform some action. For \l{Standard Shortcuts}
    {standard shortcuts}, use a standard key to request the platform-specific
    key sequence associated with each shortcut.

    For custom shortcuts, use human-readable strings, such as \key Ctrl+Q or
    \key Alt+F. You can translate them into the appropriate shortcuts for the
    speakers of different languages.

    If you hard-code keyboard shortcuts in your application, translators cannot
    override them.

    When you use keyboard shortcuts in menu item and button text, a
    \e mnemonic character (marked by underlining) indicates that pressing
    \key Alt or \key Ctrl with the underlined character performs the same action
    as clicking the menu item or pressing the button.

    For example, applications often use \e F as the mnemonic character in the
    \uicontrol {File} menu, so you can either click the menu item or press
    \key {Alt+F} to open the menu. To define the mnemonic character in the
    translatable string ("File"), prefix it with an ampersand: \c {"&File"}.
    The translation for the string should also have an ampersand in it,
    preferably in front of the same character.

    \section2 QML Example

    In QML:

    \badcode
    Menu {
        id: fileMenu
        title: qsTr("&File")

        MenuItem {
            objectName: "quitMenuItem"
            text: qsTr("E&xit")
            onTriggered: Qt.quit()
        }
    }
    \endcode

    \section2 C++: Use the QKeySequence Class

    In C++, use QAction and QKeySequence objects to specify the keyboard
    shortcuts that trigger actions:

    \code
    exitAct = new QAction(tr("E&xit"), this);
    exitAct->setShortcuts(QKeySequence::Quit);
    \endcode

    The translations of keyboard shortcuts are associated with the QShortcut
    context.

    \section1 Use Locale to Extend Localization Features

    You might find different graphics or audio more suitable for different
    geographical regions.

    Generally, try to avoid localizing images. Create icons that are globally
    appropriate, rather than relying on local puns or stretched metaphors.
    However, you might have to reverse images of left and right pointing arrows
    for Arabic and Hebrew locales.

    Locale is one of the default file selectors, so you can use file selection
    to display different images that you deliver as resources depending on the
    system locale.

    The QML and C++ code examples in the following sections assume that you
    deliver the following files in the application resources and use language
    and country codes as the subfolder names:

    \badcode
    images
    ├── language-icon.png
    ├── +en_GB
    │   └── language-icon.png
    └── +fi_FI
        └── language-icon.png
    \endcode

    \section2 QML: Set Image Source

    The following QML code snippet shows how to select an icon source image
    according to the current locale:

    \code
    icon.source: "qrc:/images/language-icon.png"
    \endcode

    \section2 C++: Use QFileSelector

    The following C++ code snippet uses QFileSelector to pick a language icon
    from the \c images folder according to the system locale:

    \code
    const QFileSelector selector;
    const QIcon languageIcon(selector.select(":/images/language-icon.png"));
    \endcode

    \section1 Enable Translation

    TS file names must contain ISO language and country codes:

    \list
        \li \e language is an \l {https://www.iso.org/iso-639-language-codes.html}
            {ISO-639} language code in lowercase.
        \li \e country is an \l {https://www.iso.org/iso-3166-country-codes.html}
            {ISO-3166} two-letter country code in uppercase.
    \endlist

    For example, \c qml_de.ts sets the target language to German, and
    \c qml_de_CH.ts sets the target language to German and the target
    country to Switzerland. The \c lrelease tool generates QM files
    called \c qml_de.qm and \c qml_de_CH.qm that the application loads
    depending on the system locale.

    \section2 QML: Use QQmlApplicationEngine

    In QML, use \l QQmlApplicationEngine to automatically load translation files
    from a subdirectory called \c i18n in the directory that contains the main
    QML file. The translation file names must have the prefix \c qml_.
    For example, \c qml_en_US.qm.

    Applications reload translations when the \l QJSEngine::uiLanguage or
    \l {Qt::uiLanguage}{Qt.uiLanguage} property value changes.

    \section2 C++: Use QTranslator

    In C++, the TS file names must contain the application name. For example,
    \c app_de_DE.ts.

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

    \snippet snippets/code/doc_src_i18n.cpp 8

    For a translation-aware application, you create a QTranslator object,
    \l {QTranslator::load()}{load} a translation according to the user's
    UI display locale at runtime, and install the translator object into
    the application.


    \section1 Prepare for Dynamic Language Changes

    Both Qt Widgets and Qt Quick use
    \l {The Event System}{Qt's event system} to inform classes about
    translation changes.

    \l{QEvent::LanguageChange}{LanguageChange} events are posted when you use
    the QCoreApplication::installTranslator() function to install a new
    translation. Other application components can also force widgets or
    QML types derived from the \l Item type to update themselves by posting
    \c LanguageChange events to them.

    By default, \c LanguageChange events are propagated to all top-level
    windows, and from there they're propagated through the entire tree of
    widgets or QML types derived from Item.

    \section2 Qt Widgets: Override changeEvent

    The default event handler for QWidget subclasses responds to the
    QEvent::LanguageChange event and calls the \c {changeEvent()}
    function when necessary.

    To make Qt widgets aware of changes to the installed QTranslator objects,
    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. For example:

    \snippet snippets/code/doc_src_i18n.cpp 12

    When using \QD UI files (.ui) and \c uic, you can read the new
    translation files and call \c ui.retranslateUi(this) directly:

    \snippet snippets/code/doc_src_i18n.cpp 17

    To pass on other change events, call the default implementation of the
    function.

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

    \section2 QML: Override event for Types Derived from Item
    For plain QML applications without any custom C++ registered types,
    \l {QML: Use QQmlApplicationEngine}{using QQmlApplicationEngine} is enough
    to trigger an update of all translation bindings.

    However, if you registered a type derived from QQuickItem, and one of
    its properties exposes translated text (or is otherwise language
    dependent), override its \l{QObject::event}{event method} and
    emit the property's change signal in it (or call
    \l{QObjectBindableProperty::notify()}{notify} in case of bindable
    properties). For example:

    \snippet snippets/code/doc_src_i18n.cpp 15

    This ensures that any binding in QML in which the property is used is
    reevaluated and takes the language change into account.

    \section2 Generic QObject-derived Classes: Use Event Filters

    Some classes are neither derived from QWidget nor from QQuickItem, but might
    still need to handle language change events. In that case, install
    \l{Event Filters}{an event filter} on
    QCoreApplication.

    \snippet snippets/code/doc_src_i18n.cpp 16

    This might be necessary when translated strings are provided by the class
    that later get displayed in a user interface (for example, a custom
    \l{QAbstractItemModel}{item model}), or when the class acts as a container
    of Widgets or Quick Items, and is therefore responsible for forwarding the
    event to them.

    \section1 Additional Considerations for C++ Code

    The following sections contain more information about using the Qt C++
    classes and functions in translatable applications:

    \list
        \li \l {Use QString for All User-Visible Text}
        \li \l {Define a Translation Context}
        \li \l {Translate Non-Qt Classes}
        \li \l {Translate Text That is Outside of a QObject Subclass}
    \endlist

    \section2 Use QString for All User-Visible Text

    QString uses the \l {Unicode in Qt}{Unicode} encoding internally, and
    therefore you can use familiar text processing operations to transparently
    process all languages in the world. Also, since all Qt functions
    that present text to the user take a QString object as a parameter,
    there is no \c{char *} to QString conversion overhead.

    \section2 Define a Translation Context

    The translation context for QObject and each QObject subclass is the
    class name itself. If you subclass QObject, use the Q_OBJECT macro in the
    class definition to override the translation context. The macro sets the
    context to the name of the subclass.

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

    \code
    class MainWindow : public QMainWindow
    {
        Q_OBJECT

    public:
        MainWindow();
    \endcode
    \dots

    If you do not use Q_OBJECT in a class definition, the context is
    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 uses the \c QWidget context if you invoke its \c tr()
    function.

    \section2 Translate Non-Qt Classes

    You must provide extra information for \c lupdate about strings in classes
    that do not inherit QObject or use the Q_OBJECT macro. To add translation
    support to a non-Qt class, you can use the Q_DECLARE_TR_FUNCTIONS() macro.
    For example:

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

    This provides the class with \l{QObject::}{tr()} functions that you can use
    to translate strings associated with the class, and enables \c lupdate to
    find translatable strings in the source code.

    Alternatively, you can call the QCoreApplication::translate() function
    with a specific context that \c lupdate and Qt Linguist recognize.

    \section2 Translate Text That is Outside of a QObject Subclass

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

    \snippet snippets/code/doc_src_i18n.cpp 13
*/

/*!
    \page i18n-plural-rules.html
    \title Translation Rules for Plural Forms
    \ingroup internationalization
    \previouspage Localizing Applications
    \brief A summary of the translation rules for plural forms produced by Qt's
    translation tools.

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

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