If people are being excluded from being able to have normal working relationships because of a language barrier, then I can see why an employer would want to put a stop to that.
It makes no difference if two people working alone in a tiny off licence or something are talking to each other in a different language, but when it means that colleagues are excluded from conversation and are made to feel awkward because they don't know what's being said, then it's rude and would create an unfriendly work environment.
Perhaps a better rule would be that all employees must be courteous and considerate to each other, which includes not talking around them in a language they can't understand.