Teams does not contain approriate lang-tags
If I have VoiceOver for Mac set to Norwegian as I usually do but my operating system to English then teams is displayed in English. However VoiceOver tries to read the English strings using a Norwegian voice, it can be very difficult to understand what the voice says when the computer trying to read English words in a Norwegian voice
Most of my chat conversation is in Norwegian, but not all. As such Microsoft Teams needs to have all content in Teams written by Microsoft inside an appropriate lang tag. Users, especially users of screen readers, should be able to set language manually. Or use some of that magical Microsoft technology to auto-detect conversation language.
In essence I want you to follow WCAG SC 3.1.1 and provide means for user generated content to get appropriate lang tags.