Internationalization with Qt

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 \
                      wackywidget.h
    SOURCES         = funnydialog.cpp \
                      main.cpp \
                      wackywidget.cpp
    FORMS           = fancybox.ui
    TRANSLATIONS    = superapp_dk.ts \
                      superapp_fi.ts \
                      superapp_no.ts \
                      superapp_se.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 $QTDIR/bin. 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

    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

    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.

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