Can I be ordered by threat of firing not to speak Spanish with fellow employees while on the job?

Federal law prohibits discrimination in employment on the basis of race, sex, religion, age over 40, or sex. Michigan law adds a number of other protected categories, including national origin; however, neither specifically protects language (that is, neither specifically says that a company may not discriminate on the basis of an employee's first or native language, though language-based discrimination would be illegal if it is actually racial or, in Michigan, national origin-based, discrimination).

However, more to the point, it is NOT discriminatory to prohibit the use of some language other than English at work (except as necessary for the job--e.g. to speak with customers). That is nothing more than a company (1) regulating its workplace for what it considers best efficiency (e.g. ensuring that all conversations are in the same language, so there are no misunderstandings); (2) avoiding what could be divisive at work (e.g. having some people speaking among themselves in Spanish could alientate or disaffect non-Spanish speakers at work); and (3) similar to 2), above, ensuring that the company is not accused of some form of illegal discrimination or harassment by letting some employees make others feel excluded.

In short, while it might be illegal to not hire someone because he or she speaks Spanish, if such action may be a front for racial or national origin discrimination, there is nothing illegal about requiring all workplace conversations to be in English. You may, of course, speak to your coworkers in Spanish after work is over, on weekends, etc. if you choose.