Document mapping of locales to loading specific .qm files

We should have a common place where the Qt logic of loading specific .qm files for locales is commonly documented in the overview documentation.

Specify Translation Sources in Qt Project Files mentions that one "Use ISO language and country codes in the TS file name to determine the language to load at runtime", but without giving details how ISO language and country code should be used.

https://doc.qt.io/qt-6/qtranslator.html#load-1 and https://doc.qt.io/qt-6/qqmlapplicationengine.html#details give some examples, but are also not very detailed. I think this deserves it's own documentation section.