She has no right to talk about her mother-in-law that way, no matter what language she’s speaking. 

Have you ever wondered if the old ladies at the Polish bakery who are talking to each other in Polish are talking trash about you? Your instincts might tell you that they’re doing that, and I’m sorry to report that your instincts are often correct. People do speak in their native language in order to talk smack without said smack being picked up on, and there’s nothing you can do about it unless you learn Polish or Spanish or Vietnamese.

I don’t really have a problem with a nail tech saying that my nail beds are weak, because they probably are! Also, it’s not as if she’s saying something negative about my character. I would have much more of a problem if someone close to me felt comfortable brazenly criticizing me in front of my face in a language I didn’t understand. My friend, who’s fluent in German, using German curse words under her breath is okay, but saying things in German she wouldn’t say to my face is completely out of line.