Conversations involving multiple participants, allowing collaboration and information sharing in Teams
It seems that multilanguage spell check is not implemented in new Teams. That is "reported" in the known issues about new Teams ( https://learn.microsoft.com/en-us/microsoftteams/new-teams-known-issues). Furthermore, it seems that no planning exists for adding this feature in the near future.
I really don't understand how they release a new Teams version with half of the old version's features implemented...