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.

This review is for a previous version of the add-on (1.5.1-signed.1-signed). 

Could you please check whether the problem persists with this development version?