Looks like has something to do with German language pack which might be installed on your side. Can you try uninstalling German language pack (not dictionary) and check if this solve the issue?

LaterEdit: if you can not uninstall German language pack because you can not open Thunderbird then try to open Thunderbird in safe-mode (no add-ons) - to do this search on your programs that option or press shift button on your keyboard and in the same time open Thunderbird