Case in point: A dishwasher at a Virginia hotel was laid off and told he'd be rehired after the hotel's remodeling. But after the reopening, the hotel refused to rehire him, citing its new rule that required all employees to speak fluent English. The U.S. Equal Employment Opportunity Commission (EEOC) sued the hotel for discrimination because of national origin. The hotel has suspended its fluency policy pending the case's resolution.
Bottom line: Require English fluency only on a case-by-case basis. Follow the EEOC guideline: Require people to speak fluent English at work only if it's needed "for the effective performance of the position." You'll stand on safest ground if you can cite objective business reasons for demanding English fluency, such as safety, effective supervision or communication with customers.