Well, possibly unrelated to all of this, but in my experience, multilingual spell-checking has gotten noticeably worse on both iOS and Windows, to the point where I had to disable auto-correct wherever that's possible (and that's, unfortunately, pretty far from everywhere!).

This particular problem manifests as: you're conversing in one language (say, French) and then use a single English word, at which point the spell-check and auto-correct permanently switches to that language, mis-correcting pretty much everything from that point onward.

(Classic) Outlook on Windows is pretty much entirely broken for me these days (even if I repeatedly mark the entire message as being in the majority language), as is Safari on MacOS: even in a completely-Dutch conversation, it always insists on auto-completing 'lang' ('long' but can also be 'tall') to 'language' and it's absolutely infuriating, and with no apparent way to disable the madness... (and, interestingly, no mechanism to detect that I dismissed the auto-complete for the 100th consecutive time, and that it's possibly not a desirable substitution)