We can’t speak our native language at work?

Hello! I was catching up with my old coworker and she told me that the manager told them that they weren’t allowed to speak Spanish at work. Is this legal? There’s four people that work there and two are native Spanish speakers. I found this bizarre. Is this legal? When I worked there I also spoke Spanish since it was easier but it never was disrespectful or disruptive. The patients never had an issue. We also translated. I’m just so confused. It’s not a written rule she just told them.