Load translations for application-specific language also on Win and Mac
On Windows and Apple OSs, we cannot use QLocale::system() because on those OSs Qt ignores LANGUAGE (which is set by kxmlgui to the application-specific languages configured by the user). We need to check LANGUAGE ourselves.
This MR replaces kxmlgui!145 (closed). It fixes the bug for the translations provided by Qt. To fix the bug also for translations coming from .qm files in KF libraries a similar change in ecm is needed.
I have tested this MR (for KF5) with Kate on Windows.