The lawsuit discussed by Brenda, below, illustrates the dangers of allowing your employees to chatter away in a foreign language. The point here is not only that women were insulted by Hispanic employees, but that the employees' conduct has resulted in a $4.5 million dollar lawsuit. That's why many employers have instituted an English-only policy, specifically to prevent sexual harassment. As I wrote when Ruben Navarrette Jr, [Send him mail] was gloating over an EEOC case forbidding the Salvation Army from requiring English in the workplace.

[E]mployers are responsible, under another section of EEOC's rules, the EEOC sexual and national origin harassment regulations, for their employees' speech.

See Eugene Volokh's article What Speech Does "Hostile Work Environment" Harassment Law Restrict? for the gory details of what speech employers are supposed to be monitoring and preventing.

Any racial slur or sexual harassment by an employee can result in fines or liability for an employer. How is the employer supposed to protect himself from this if he can't tell what they're saying? And of course, since Mexico has lower standards of what constitutes sexual harassment or racism than Americans do, the danger that a low-level Spanish speaking employee is saying something offensive is fairly severe.