Qt Quick should use QLocale::uiLanguages() for loading translations

qmlscene, qml, but also QQmlApplicationEngine do allow automatic loading and installation of .qm files for the respective UI. This is done by checking QLocale::system().name().

There are two problems with that:

• QQmlApplicationEngine ignores any locale enforced by QLocale::setDefault()
• QLocale::system() is actually specifying the decimal/grouping characters and date/time formats specified in the system configuration panel, not the UI language.

The QLocale documentation therefore recommends using QLocale::uiLanguages() instead, by using the convenience method QTranslator::install(QLocale, ...)

One problematic side-effect about changing this though is that it ignores the language of the translation sources; that is, if the sources are in English, but there's no en.qm translation file, it might be that a secondary language translation is loaded.

Anyhow, for Qt we have been providing empty English translations, and should endorse our users to do so, too.