Draft: Make override language work properly on Windows
Set LANG instead of LANGUAGE on Windows and macOS
This fixes the problem that the application-specific language wasn't used for Qt translations on Windows (and most likely also on macOS). On Windows QLocale (Qt 5 and Qt6) only looks on LANG. On macOS Qt 5 looks only on LC_ALL, LC_MESSAGES, and LANG while Qt 6 uses macOS core foundation libraries. So, most likely this fix won't work on macOS with Qt 6.
Note: Since this fix will probably not work on macOS with Qt 6, I'm wondering whether the || defined(Q_OS_OSX) makes sense. On macOS with Qt 5 it should work (but only if neither LC_ALL nor LC_MESSAGES is set). So maybe the || defined(Q_OS_OSX) doesn't even make sense for KF5/Qt 5.