When you write messages in different languages, you’ll see spell checking relevant to the language you’re actively using while composing your message.
Write several messages in a chat or channel in a different language to activate the language-aware spell checking automatically. In this case, you may see a message asking you to confirm which language you’re currently using.
Switch between languages that use different keyboard layouts by using the Language bar. Your Windows keyboard language is what controls the default spell check language in Teams. Click the language icon on the Language bar, which should appear on your taskbar in Windows near where the clock is, and then click the language that you want to use.
If the above method doesn't work, try disabling Teams spell checking and re-enabling it (this method requires restarting Teams).
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