Yeah, the automatic detection of language has never been perfect. The flaws are best seen in Word, which is why I would never recommend using the feature in that program.
Here, when I test a new e-mail message in Outlook 2021, the automatic detection is working rather well. It works better after I have pressed Enter to begin a new paragraph, though. As you can tell, no wavy underlines indicating spelling errors are shown...

I should clarify that if I type a single word in English, surrounded by Swedish text, then the automatic detection may fail. All text will be marked as Swedish and the English word will be flagged as incorrect.