As with all company work rules, an English-only policy can only be implemented by your boss or job for nondiscriminatory reasons only. To that end, your manager cannot create an English-only rule with the intent to discriminate against employees of a particular national origin.
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 consequence of violating it.