Using Windows Desktop Application.
I have my language in the app set to English, yet I still get spelling autocorrect in German (really bad suggestions as well) because that's my windows language.
The browser has no trouble with this, it even shows English and German suggestions both.
Still the suggestions are not stelar but at least I don't get my whole text underlined because I write in English.
The Web version (screenshot) is just what I need for desktop