The question comes up frequently as to whether an employer can enforce an “English-only” rule at work. Here is what the EEOC says about it:

The EEOC has stated that rules requiring employees to speak only English in the workplace violate the law unless they are reasonable necessary to the operation of the business.
A rule requiring employees to speak only English in the workplace at all times, including breaks and lunch time, will rarely be justified.
An English-only rule should be limited to the circumstances in which it is needed for the employer to operate safely or efficiently.
Circumstances in which an English-only rule may be justified include: communications with customers or coworkers who only speak English; emergencies or other situations in which workers must speak a common language to promote safety; cooperative work assignments in which the English-only rule is needed to promote efficiency.
Even if there is a need for an English-only rule, an employer may not take disciplinary action against an employee for violating the rule unless the employer has notified workers about the rule and the consequences of violating it.