Problem statement
In cross-language teams collaboration it is essential that everyone tries to keep the conversation accessible for everyone. Sometimes group chats start from one language and some time later others join the chat forcing already joined people to switch to language that's clear for everyone (English usually). Other cases could be defined as requiring participants to use the only allowed language for communication.

To address those concerns I propose to have following improvements:

1. If the discussion was happening for some time already, it requires newcomers to translate all the prior messages one by one. So, in this case it would be nice to have an option to do translation of all existing messages to the new common language of the chat. Still, those messages should have an option to show original message content. The action has to be manually triggered by a group admin.

2. Even when some chat group has been declared as a "Some particular language only" (English-only, etc), some people may still use other languages. In this case, it would help to make either "soft" and/or "hard" constraint enforcing the rule. For the soft constraint it would warn users if they're trying to post something in non-allowed language, and for the hard one it has to forbid doing so. For the implementation purpose, this could either use alphabet-based heuristics, or a lightweight AI classifier, or even both.

3. It would be nice to indicate in the UI that some groups have preferred languages for communication, or have language restrictions.