Use KLanguageName instead of kf5_all_languages

kf5_all_languages is part of kdelibs4support and will eventually disappear

I thought of adding an ifdef to not increase the KF5 dependency, but since it was already at 5.71, i don't think increasing it to 5.74 is too bad.

Anyhow i think we should wait before 5.74 is released before commiting this to git