Legal Question in Employment Law in California

That is not an issue you should decide on the basis of some free hints and tips from here, or anywhere. It invokes a discussion of Title VII discrimination rules and violations that carry substantial penalties. YOu should consult in depth with employment law counsel before making any decisions. Feel free to contact me if serious about doing so. You may be able to require English only at work, and may be able to use that as the pre-condition to offering employees time to become fluent or be terminated.