Using Classic Outlook on Windows for personal email, calendar, and contact management
My default spellchecking language is Polish but I often write in English or German. To switch the spellchecker to the other language I close the draft and re-open it - it seems to cause the spellchecker to detect the different language and work properly.
Of course this is only a workaround and it's infuriating that Microsoft is not able to fix such a basic issue in one of their most used (and paid for) applications...