I could see an argument that it creates cliques and can cause issues between coworkers, especially since they may feel safe gossiping about other coworkers or things like that.

However, I don’t agree with that. All of that can happen with everyone speaking English. I don’t think it’s an issue. I think potentially what it could be is the boss not wanting them to be able to communicate discretely. It seems like something that could be an anti-unionization move. Maybe I’m being too cynical, but that’s often what these weird rules that don’t seem logical seems to be.