Some translation file does not work

When I load the qt_fr file located in QLibraryInfo :: location (QLibraryInfo :: TranslationsPath). The buttons are not translate in French

I wrote

translator.load (QString ("qt_fr") QLibraryInfo :: location (QLibraryInfo :: TranslationsPath));

app.installTranslator (& translator);

BUT

translator.load (QString ("qt_de") QLibraryInfo :: location (QLibraryInfo :: TranslationsPath));

app.installTranslator (& translator);

works ..

qt_fr dosn't work, but qt_de work..

Attachment, I put a test code