Language-Aware Spellchecking in Teams

The Windows desktop client for Teams monitors text as users type chat and channel messages to detect if they switch language. And if they do, Teams can change language for spell checking. The code runs on the client and no data is transmitted back to base. It’s all very intelligent, but you should warn users that it might happen.

https://office365itpros.com/2020/07/10/teams-switch-spell-check-language/