I'm French and I use my Mac and all apps in their English versions.
In Teams, when I write a message in French, everything is underlined in red because of the spelling/grammar check that is by default in English...
It could be very very convenient to have the ability to set the language in Teams and avoid using the same language as the OS installation.
Spell check for non English is not yet there ...
Please check the Teams user voice ... and add your voice there..
for selecting your language from the teams settings
Hope this helps,