As much as I embrace immigration in general and diversity in the workplace, I agree with pohnpei's response. What I do want to point out is that the question is framed as though presently corporations are not allowed to do this, but to the best of my knowledge, there is no law right now preventing them from doing so, and in fact, I am not aware of any statute that prevents even a government office from doing the same. Certainly, there are many reasons that government in particular should not do this, the same reasons that have precluded the United States from having an official language, but the fact remains, in spite of the Senate having voted in 2006 on the matter of English as the official language, there is still, as far as I know, no law at all. Canada, on the other hand, has a statute that promotes a policy of diversity in language.
Of course, many people do not agree with this point of view. There are many who would say that English-only rules in the workplace are insulting to people whose native language is not English. They would say that these rules are discriminatory and unnecessary.
However, if we need to argue that these rules should be acceptable, there are two main ways to do so. First, we should point out that we are talking about private businesses, not about governmental institutions. As long as businesses are willing to hire people of all races and all ethnic groups, they should be allowed to make whatever reasonable rules they want to govern those people’s behaviors. If people do not like the rules the business imposes, they should work somewhere else. Second, it can be argued that English-only rules are reasonable in many cases. Businesses can argue that their customers are put off when they hear employees speaking in foreign languages. It makes them feel unwelcome and it makes them wonder what is being said. If employees are acting in ways that make customers feel uncomfortable, we can argue, it is the right of businesses to impose rules that will make them stop doing so.