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Internationalization with Qt

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

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.

See also the Qt Linguist manual.

Step by Step

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

Use QString for all User-visible Text

Since QString uses the Unicode 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 char* to QString conversion time overhead.

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

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

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 QApplication::translate() function. Essentially all that is necessary to achieve this is to use QObject::tr(). For example, assuming the LoginWidget is a subclass of QWidget:

        QLabel *label = new QLabel( tr("Password:"), this );

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 QApplication::translate() function directly:

    void some_global_function( LoginWidget *logwid )
        QLabel *label = new QLabel(
                LoginWidget::tr("Password:"), logwid );

    void same_global_function( LoginWidget *logwid )
        QLabel *label = new QLabel(
                qApp->translate("LoginWidget", "Password:"),
                logwid );

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 lupdate utility described below. The macros expand to just the text (without the context).

Example of QT_TR_NOOP():

    QString FriendlyConversation::greeting( int greet_type )
        static const char* greeting_strings[] = {
            QT_TR_NOOP( "Hello" ),
            QT_TR_NOOP( "Goodbye" )
        return tr( greeting_strings[greet_type] );


    static const char* greeting_strings[] = {
        QT_TRANSLATE_NOOP( "FriendlyConversation", "Hello" ),
        QT_TRANSLATE_NOOP( "FriendlyConversation", "Goodbye" )

    QString FriendlyConversation::greeting( int greet_type )
        return tr( greeting_strings[greet_type] );

    QString global_greeting( int greet_type )
        return qApp->translate( "FriendlyConversation",
                                greeting_strings[greet_type] );

If you disable the const char* to QString automatic conversion by compiling your software with the macro QT_NO_CAST_ASCII defined, you'll be very likely to catch any strings you are missing. See QString::fromLatin1() for more information. Disabling the conversion makes programming cumbersome.

If your source language uses characters outside Latin-1, you might find QObject::trUtf8() more convenient than QObject::tr(), as tr() depends on the QApplication::defaultCodec(), which makes it more fragile than QObject::trUtf8().

Use QKeySequence() for Accelerator Values

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

    QPopupMenu *file = new QPopupMenu( this );
    file->insertItem( tr("&Quit"), this, SLOT(quit()),
                      QKeySequence(tr("Ctrl+Q", "File|Quit")) );

Use QString::arg() for Simple Arguments

The printf() style of inserting arguments in strings is often a poor choice for internationalized text, as it is sometimes necessary to change the order of arguments when translating. Nonetheless, the QString::arg() functions offer a simple means for substituting arguments:

    void FileCopier::showProgress( int done, int total,
                                   const QString& current_file )
        label.setText( tr("%1 of %2 files copied.\nCopying: %3")
                        .arg(current_file) );

Produce Translations

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

Qt Linguist's manual provides further information about Qt's translation tools, Qt Linguist, lupdate and lrelease.

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

  1. Run 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 QT_*_NOOP macros described above and produces .ts files (usually one per language).

  2. Provide translations for the source texts in the .ts file, using Qt Linguist. Since .ts files are in XML format, you can also edit them by hand.

  3. Run 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.

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

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

    HEADERS         = funnydialog.h \
    SOURCES         = funnydialog.cpp \
                      main.cpp \
    FORMS           = fancybox.ui
    TRANSLATIONS    = superapp_dk.ts \
                      superapp_fi.ts \
                      superapp_no.ts \

When you run lupdate or 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 qmake (or tmake), you usually don't need an extra project file for lupdate; your qmake project file will work fine once you add the TRANSLATIONS entry.

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

If you have been using the old Qt tools (findtr, msg2qm and mergetr), you can use qm2ts to convert your old .qm files.

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

While these utilities offer a convenient way to create .qm files, any system that writes .qm files is sufficient. You could make an application that adds translations to a QTranslator with QTranslator::insert() and then writes a .qm file with QTranslator::save(). This way the translations can come from any source you choose.

Qt itself contains about 400 strings that will also need to be translated into the languages that you are targeting. You will find translation files for French and German in $QTDIR/translations as well as a template for translating to other languages.

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

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

        // translation file for Qt
        QTranslator qt( 0 );
        qt.load( QString( "qt_" ) + QTextCodec::locale(), "." );
        app.installTranslator( &qt );

        // translation file for application strings
        QTranslator myapp( 0 );
        myapp.load( QString( "myapp_" ) + QTextCodec::locale(), "." );
        app.installTranslator( &myapp );


        return app.exec();

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:

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

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

    ...; // use encoded_string in 8-bit operations

For converting Unicode to local 8-bit encodings, a shortcut is available: the local8Bit() method of QString returns such 8-bit data. Another useful shortcut is the utf8() method, which returns text in the 8-bit UTF-8 encoding: this perfectly preserves Unicode information while looking like plain US-ASCII if the Unicode is wholly US-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:

    QCString encoded_string = ...; // Some ISO 8859-5 encoded text.

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

    ...; // Use string in all of Qt's QString operations.

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()).

Since most Unix systems do not have built-in support for converting between local 8-bit encodings and Unicode, it may be necessary to write your own QTextCodec subclass. Depending on the urgency, it may be useful to contact Trolltech technical support or ask on the qt-interest mailing list to see if someone else is already working on supporting the encoding. A useful interim measure can be to use the QTextCodec::loadCharmapFile() function to build a data-driven codec, although this approach has a memory and speed penalty, especially with dynamically loaded libraries. For details of writing your own QTextCodec, see the main QTextCodec class documentation.


Localization is the process of adapting to local conventions such as date and time presentations. Such localizations can be accomplished using appropriate tr() strings, even "magic" words, as this somewhat contrived example shows:

    void Clock::setTime(const QTime& t)
        if ( tr("AMPM") == "AMPM" ) {
            // 12-hour clock
        } else {
            // 24-hour clock

Localizing images is not recommended. Choose clear icons that are appropriate for all localities, rather than relying on local puns or stretched metaphors.

System Support

Operating systems and window systems supporting Unicode are still in the early stages of development. The level of support available in the underlying system influences the support Qt provides on that platform, but applications written with Qt need not generally be too concerned with the actual limitations.



Supporting More Input Methods

While Trolltech doesn't have the resources or expertise in all the languages of the world to immediately include support in Qt, we are very keen to work with people who do have the expertise. Over the next few minor version numbers, we hope to add support for your language of choice, until everyone can use Qt and all the programs developed with Qt, regardless of their language.

Languages with single-byte encodings (European Latin-1 and KOI8-R, etc.) and multi-byte encodings (East Asian EUC-JP, etc.) are supported. Support for the "complex" encodings: those requiring right-to-left input or complex character composition (e.g. Arabic, Hebrew, and Thai script) is implemented, but the range of Indic scripts (Hindi, Devanagari, Bengali, etc.) is still under development. The current state of activity is:

Encodings Status
All encodings on Windows The local encoding is always supported.
ISO standard encodings ISO 8859-1, ISO 8859-2, ISO 8859-3, ISO 8859-4, ISO 8859-5, ISO 8859-7, ISO 8859-9, and ISO 8859-15 Fully supported.
KOI8-R Fully supported.
eucJP, JIS, and ShiftJIS Fully supported. Uses eucJP with the XIM protocol on X11, and the IME Windows NT in Japanese Windows NT. Serika Kurusugawa and others are assisting with this effort. kinput2 is the tested input method for X11.
eucKR Supported. Mizi Research are assisting with this effort. hanIM is the tested input method.
Big5 Qt contains a Big5 codec developed by Ming Che-Chuang. Testing is underway with the xcin (2.5.x) XIM server.
eucTW Under external development.

More information on the support of different writing systems in Qt can be found in the documentation about writing systems.

If you are interested in contributing to existing efforts, or supporting new encodings beyond those mentioned above, your work can be considered for inclusion in the official Qt distribution, or just included with your application.

Eventually, we hope to help Unix become as Unicode-oriented as Windows is becoming. This means better font support in the font servers, with new developments like the True Type font servers xfsft, xfstt, and x-tt, as well as UTF-8 (a Unicode encoding) filenames such as with the Unicode support in Solaris 7.

Note about Locales on X11

Many Unix distributions contain only partial support for some locales. For example, if you have a /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 that /usr/share/locale/ja_JP.EUC directory needs to be complete. For best results, use complete locales from your system vendor.

Relevant Qt Classes

These classes are relevant to internationalizing Qt applications.

QBig5CodecConversion to and from the Big5 encoding
QEucJpCodecConversion to and from EUC-JP character sets
QEucKrCodecConversion to and from EUC-KR character sets
QGb18030CodecConversion to and from the Chinese GB18030/GBK/GB2312 encoding
QGb2312CodecConversion to and from the Chinese GB2312 encoding
QGbkCodecConversion to and from the Chinese GBK encoding
QHebrewCodecConversion to and from visually ordered Hebrew
QJisCodecConversion to and from JIS character sets
QSjisCodecConversion to and from Shift-JIS
QTextCodecConversion between text encodings
QTextDecoderState-based decoder
QTextEncoderState-based encoder
QTranslatorInternationalization support for text output
QTranslatorMessageTranslator message and its properties
QTsciiCodecConversion to and from the Tamil TSCII encoding

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Qt version 3.1.1