Sometimes I get red underlining that I don't understand. E.g. here after inserting the Google Translate result via a keyboard shortcut. These segments will be flagged in QA spelling.
I get these too. It's from Google Translate. I sometimes wonder if they are marking their MT output somehow. Just select/copy/paste the relevant text into Word, and switch on Show hidden characters. I think they are non-breakable spaces or something like that. Never really checked, I just always delete them.