Hard for me to duplicate (so I haven’t yet logged a bug in YouTrack) but somehow one of installed language packs (Polish) got deleted presumably upon recent Rhino 6 update (from 6.0 to 6.1?).
Moreover, now in Win 10 <Apps & features - Rhino 6 - Modify - Modify> no language is listed but there should be English at least:
Maybe other translators have observed it? Or @Helvetosaur?