Thanks. Imagine the law required then to allow her to make the calls in Spanish. They'd have to hire a Spanish speaker to monitor the calls. Typically, when you hire a foreign language speaker, they demand more pay than native speakers, so the company would need to offer more pay for that monitor to find a good hire. But what if the next caller employee speaks Russian. Now, the company needs to find a Russian speaking monitor for quality assurance and to ensure the caller employee is properly performing their job. What if the caller employee speaks Swahili? Eventually the company, especially if it isn't located in a major metropolitan area, is going to find it very difficult to hire call monitors who speak whatever language the caller employee happens to speak.
It would place an incredible burden on the company, IMO. They should be allowed to dictate that all business be conducted English, as it would limit their liability and otherwise be nearly impossible for them to hire monitors that speak potentially any language spoken.