Have you experienced people trying to always build / maintain monoethnic teams? How do you approach this problem and how do you even talk about it?
This is very specific to my EU/Singapore experience so I am not sure how big of a problem this is in the US/North America.
Even though the working language is english there's usually teams where one person decides to always hire people of his ethnicity or people that speak a specific language.
Meetings with such team is always awkward as they internally discuss the topic in their own language during the meeting. Also their hiring practices are essentially discriminatory.
To help this, there was a norm where, if something needed to get done quicker or some technical point was not getting across, the Chinese eng manager would say "OK, we are going to speak Chinese for a minute" and the team would deliberate some point in their native language. Then, when it was resolved, we'd continue on in English with the better English-speakers summarizing. This seemed to work pretty well, and was not awkward for anyone. It allowed the majority-Chinese team to contribute as full team-members regardless of their English proficiency, and also allowed the monolingual US minority to contribute equally.
For OP, it might be good to suggest leadership sits down and figures out good "rules of the road" for language use in the office, and to make sure they enforce them.