It's also - in most cases - very wrong.
The EEOC provides some examples where English-only would be justified by business necessity:
- For communications with customers, co-workers, or supervisors who only speak English.
- In emergencies or other situations in which workers must speak a common language to promote safety.
- For cooperative work assignments in which the English-only rule is needed to promote efficiency.
- To enable a supervisor who only speaks English to monitor the performance of an employee whose job duties require communication with co-workers or customers.
It's not worth it!