When I use German words for my variable names and these include umlauts or special characters, I write those out so the names are completely written in the normal latin alphabet. ReShaper marks those words as wrong because it can't detect this type of spelling.
I've tried looking under ReSharper Options > Grammar and Spelling > Grammar > and searched for both "umlaut" and "Sonderzeichen" there, but didn't find any options. I'd love if ReSharper could detect written out special characters some way. This is what I mean by writing them out:
I currently have 3 German dictionaries installed. The one that came with ReSharper and two I added later.
ReSharper doesn't have such a feature now. You are welcome to vote for this request in YouTrack. Thank you!