There are a few issues here. First, Houston has a high population of Mexican immigrants. Is it possible this particular Walmart is located in a part of town that has a concentrated immigrant population? If this is the case, it would explain why Spanish was being played over the speakers.

Second, I don't think Walmart is catering to Mexicans so much as they are catering to their customer. If that location's customer tends to be Hispanic, it would make sense that Spanish would be the dominating language in this store. By the way, I have no problem with making English the official language of the US, but I don't see why it should be English-only-all-the-time. There are plenty of countries that have an official language (while still acknowledging the high populations of other nationalities either visiting or living in their country) and even some countries that have multiple official languages.

Third, English is a very hard language to learn, one of the hardest in the world from what I understand. It's unfair to make the leap that because someone of Mexican descent isn't speaking it fluently that they don't want to learn how.

However, I do agree that if they have a problem with English they shouldn't be working with the public. I don't know if this was the case with this particular cashier, but if it is, high Hispanic population or not, she shouldn't be in the position she's in.