Message translation is now available in Microsoft Teams. (Click the screenshots below to see larger versions.)
To translate a message, simply click on the ellipsis icon (the three dots just to the right of the emojis) and click “Translate.”
The message will automatically be translated to the language you have specified as the default language for Teams.
To set a default language, simply go to Settings > General > Language.
View a list of supported languages in this article from Microsoft.