An employer can impose rules requiring that only English be spoken in the work place.  But, an employer can do so only if there is a real, definable business reason for the rule.  Most cases that have allowed such English-only rules were based on safety reasons.  See post.  Of course, no such rule could be imposed if the primary purpose is discrimination.  But, if safety or some other business reason is not involved, it will appear that discrimination is the motivation.  If so, then Title VII of the Civil Rights act of 1964 will apply.