Warren Christianson owns a chain of sandwich shops in Arizona. $60\%$ of his workforce are immigrants from Central America an they often speak Spanish while at work. They always speak English when serving customers $($unless the customer indicates that they would prefer to order in Spanish$).$ Warren is concerned that when they are speaking Spanish his employees are wasting time discussing non$-$work$-$related matters and may even be saying disparaging comments about him. So he has decided to require his employees to speak only English at work. Is he permitted to do this?