[user104658]

It depends.

If they're speaking to each other in their native language just so that they can better communicate an idea or topic to each other that way then there's obviously absolutely no problem with it.

If they're doing it specifically because they don't want other people who are around to know what they're, then it's quite rude. Worse still is if they're quite blatantly talking / laughing about someone or something and pointedly want to keep others out of the loop.