load qt_ja/pm use QTranslator.

main.cp

#include <QtGui>
#include "mainwindow.h"

int main(int argc, char * argv[])
{
    QApplication app(argc, argv);
    QString displayLanguage = QLocale::system().name();
    QTranslator *translator = new QTranslator(&app);
    translator->load(QString("qt_%1").arg(displayLanguage), QLibraryInfo::location(QLibraryInfo::TranslationsPath));
    app.installTranslator(translator);

    QTextCodec::setCodecForTr(QTextCodec::codecForLocale());

    MainWindow *browser = new MainWindow(app);
    browser->show();
    return app.exec();
}