wow, that IS rude! ugh.
i had this problem with CNAs speaking Spanish all the time. Now, I don't speak Spanish aside from a few common phrases, but I could understand a lot more than they thought, because I speak fluent Italian which is probably the closest romance language to Spanish.
The CNAs would switch to Spanish when they wanted to ***** and talk trash about other people. One day two of them were helping me put a 400-lb woman on a bedpan, and they started calling her a pig and saying how disgusting she was and that you'd have to pay someone a lot of money to have sex with her.
The look on their faces was freaking hilarious when i said to them - in Spanish! - 'you know i understand everything you say, right?' they looked like they were about to **** themselves!
Then I lied and told them (in English) that almost everyone who works here understands Spanish, and that *everyone* has been hearing and understanding everything they've ever said to each other on the floor. One of them turned so white i thought she was going to pass out. Since then, they've magically decided it would be in their best interests to speak English at work. :)