The default language for my Office suite is English (UK). When I start typing in a different language, say Dutch, the spellings checkers corrects Dutch text I have entered to a "correct" English word.
After I have completed some part of my text (in general a sentence) the automatic language detection switches the language settings to Dutch and will use the Dutch dictionary to correct my input. It now detect the previous corrections made to be wrong.
In my opinion the proper way would be to first undo all corrections before switching to the new language settings.
I have the same problem. I am in Switzerland and can write in many languages but my software and computer are in English. The desktop applications seem to work fine but Office 365 online does not. I have also noticed that the browser settings seem to change at random. I had enabled spell checking (Edge Canary) in English, French, German and Italian, but just now only French was left so this entire message was highlighted. But even if I open Office in Chrome - and chrome does detect language properly, Outlook and Word online get it wrong. Oh so painful. We did this 30 years ago and now we start again. Please also look at date formats in Planner and Excel insdise Teams if anyone is paying attention. I will post in the Canary feedback as they may be more attentive.