If all chat participants share the same language on their Microsoft account, and that language is supported, then the bot sends alerts in that language.

If not, the language set for the tenant in the Control Panel will be used.

Note: If you have made a language change on your Microsoft account that is not being reflected in the bot, please note that there is a cache on Microsoft's side that has a variable lifetime (we have seen 24 hours, sometimes more) and that stores the language information for each user, so the change may take some time to propogate.