Can My Employer Ban Me From Speaking Spanish To Co-Workers?

Good afternoon, I have a question. I was just told by my supervisor that I cannot speak Spanish to my coworkers in our department. She states that some other non-Spanish speaking workers claim it makes them uncomfortable. I am asked to assist Spanish-speaking customers with no additional pay, but this is not a concern. I will certainly comply with this request, but I am not sure if it is fair or even legal. I hope you can clarify this for me. Thank you for your assistance.