I was not able to find “Player can’t be challenged” in the codebase, which is why I assumed you meant “Already playing a game against this person”. Could you post a screenshot of the text you are seeing?
As for the Uhr startet… you’re absolutely onto something- for some reason we are converting to lower case:
ret = interpolate(_("Fischer: Clock starts with %s and increments by %s per move up to a maximum of %s."), [
There may be good reason for this (in case these time settings are stored as “2 DAYS” or something), so I don’t think I can promise a fix right now, but I’m looking into it. Good find and thanks for the hard work on translations!