I have this issue for quite a while now. Sometimes it happens sometimes it doesn’t. I have the german language-pack installed however every once in a while after an update my language is set back to english. I searched around a bit but didn’t find a solution. On my other system (also using EOS) that doesn’t happen at all but i can’t tell what’s the difference and why it would happen. Anyone got a clue?
Is seems you are not alone.
It seems it can be caused by the profile, which may have become corrupted.
Perhaps these threads are useful: