Internationalization with Qt

These classes support internationalizing of Qt applications.

• QCollator: The QCollator class compares strings according to a localized collation algorithm.

• QCollatorSortKey: The QCollatorSortKey class can be used to speed up string collation.

• QLocale: The QLocale class converts between numbers and their string representations in various languages.

• QStringConverter: The QStringConverter class provides a base class for encoding and decoding text.

• QStringDecoder: The QStringDecoder class provides a state-based decoder for text.

• QStringEncoder: The QStringEncoder class provides a state-based encoder for text.

• QTextCodec: The QTextCodec class provides conversions between text encodings.

• QTranslator: The QTranslator class provides internationalization support for text output.

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 encoding and presentation conventions.

Qt tries to make internationalization as painless as possible for developers. All input controls 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:

• All East Asian languages (Chinese, Japanese and Korean)

• All Western languages (using Latin script)

• Arabic

• Cyrillic languages (Russian, Ukrainian, etc.)

• Greek

• Hebrew

• Thai and Lao

• All scripts in Unicode 6.2 that do not require special processing

• Bengali

• Burmese (Myanmar)

• Devanagari

• Gujarati

• Gurmukhi

• Kannada

• Khmer

• Malayalam

• Tamil

• Telugu

• Tibetan

The list above is supported and will work on all platforms as long as the system has fonts to render these writing systems installed.

On Windows, Linux and Unix with FontConfig (client side font support) the following languages are also supported:

• Dhivehi (Thaana)

• Syriac

• N'Ko

On macOS, the following languages are also supported:

• Oriya

• Sinhala

Many of these writing systems exhibit special features:

• 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.

• 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 Unicode Technical Annex #9.

• 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.

• 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.

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 controls (e.g. QLineEdit, QTextEdit, and derived classes or the Quick TextInput item) and Qt's display controls (e.g. QLabel and Qt Quick's Text item).

Support for these writing systems is transparent to the programmer and completely encapsulated in Qt's text engine. This means that you don't usually need to have any knowledge of 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, 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.

For more information about how to internationalize source code, see Writing Source Code for Translation and Internationalization and Localization with Qt Quick.

Qt provides excellent support for translating Qt C++ and Qt Quick applications into local languages. Release managers, translators, and developers can use the Qt translation tools to accomplish their tasks.

The Qt translation tools, Qt Linguist, lupdate, and lrelease are installed in the bin subdirectory of the base directory Qt is installed into. For more information about using them, see the Qt Linguist Manual.

Qt itself contains several thousands of strings that will also need to be translated into the languages that you are targeting. You will find a number of translation files in the qttranslations repository. Before you start translating Qt, read the wiki page Translating Qt Into Other Languages.

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

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    QTranslator myappTranslator;
    if (myappTranslator.load(QLocale::system(), u"myapp"_s, u"_"_s, u":/i18n"_s))
        app.installTranslator(&myappTranslator);

    return app.exec();
}

For a translation-aware application, a QTranslator object is created, then a translation is loaded according to the user's preferred UI display locale at runtime, and finally, the translator object is installed into the application.

You can also use QLibraryInfo::path() to locate the translations for the Qt modules used. You can request the path to the translations at run-time by passing QLibraryInfo::TranslationsPath to this function.

Available Catalogs▲

Example: Basic Qt Modules▲

    QTranslator qtTranslator;
    if (qtTranslator.load(QLocale::system(), u"qtbase"_s, u"_"_s,
                          QLibraryInfo::path(QLibraryInfo::TranslationsPath))) {
        app.installTranslator(&qtTranslator);
    }

As of Qt 6, it is assumed that 8-bit UTF-8 is the predominant encoding.

QString::toUtf8() returns the text in UTF-8 encoding; this perfectly preserves Unicode information while looking like plain ASCII if the text is wholly ASCII.

For converting Unicode to local 8-bit encoding, a shortcut is available: the QString::toLocal8Bit() function returns such 8-bit data. On Unix systems this is equivalent to toUtf8(), on Windows the systems current code page is being used.

For converting the other way, there are the QString::fromUtf8() and QString::fromLocal8Bit() convenience functions.

For QTextStream, QTextStream::setEncoding() can be used to set common encodings.

Should other legacy encodings be required, the QTextCodec class from the Qt5Compat module can be used.

When an application starts, the locale of the machine will determine the 8-bit encoding used when dealing with external 8-bit data. QTextCodec::codecForLocale() returns a codec that can be used to convert between this locale encoding and Unicode.

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:

QString string = ...; // some Unicode text

QTextCodec *codec = QTextCodec::codecForName("ISO 8859-5");
QByteArray encodedString = codec->fromUnicode(string);

For converting the other way, the below code demonstrates the conversion from ISO 8859-5 Cyrillic to Unicode:

QByteArray encodedString = ...; // some ISO 8859-5 encoded text

QTextCodec *codec = QTextCodec::codecForName("ISO 8859-5");
QString string = codec->toUnicode(encodedString);

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

Localization is the process of adapting to local conventions, for example presenting dates and times using the locally preferred formats. For localized numbers, dates, times and currency strings, 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.

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 QTranslator objects, reimplement the widget's changeEvent() function to check whether the event is a LanguageChange event, and update the text displayed by widgets using the tr() function in the usual way. For example:

void MyWidget::changeEvent(QEvent *event)
{
    if (event->type() == QEvent::LanguageChange) {
        titleLabel->setText(tr("Document Title"));
        ...
        okPushButton->setText(tr("&OK"));
    } else
        QWidget::changeEvent(event);
}

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 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.

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.

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.

Unix/X11▲

Linux▲

Windows▲

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