Rated 4 out of 5 stars
htamas: this issue is definitely related to StartupMaster: removing this addon fixes the language support again, so that "Inbox" becomes "Posteingang" again, etc.
Re-Installing this addon turns "Posteingang" into "Inbox" again.
This behaviour is repeatable as often as you like.
Could you please check whether the problem persists with this development version?