I'm having this problem too. Unfortunately my (temporary) solution was to just punch in my password for an hour until it worked.
IIRC a friend of mine had this problem, and he said it was fixed by deleting one of the cache folders. It may have been the depotcache folder, but I'm not sure.
Have you tried another system
Do you have a spare router by chance/ tried resetting that?
You said Steam popped up in German, have you checked your internet settings and made sure your internet isn't being routed through a german proxy stealing all of your infos?
no idea. how can i check?