Your employer generally CAN require you to speak in a different language to customers if it would make the customer feel more comfortable. This is considered a ‘customer preference’ and since it is connected to the business (i.e. the customer’s ability to communicate their needs effectively), this will most likely be allowed.
Requiring you to speak English to your co-workers, however, may violate the law. The EEOC Guidelines, 29 C.F.R. 1606.7, state that blanket English-only policies are presumed to violate Title VII, which prohibits discrimination based on national origin. As the guidelines state: