Sorry I can’t agree with this.
Sometimes, the best way to express yourself is in a different language. There’ll be something that is just funnier or more serious in my parents language, I’ll often mix it with English and because I do it so much my friends who don’t speak it have picked up stuff. It’s like one of the Real Housewives shows, they often flit between Spanish and English and nobody bats and eyelid because in Miami so many people are bilingual.
Obviously if you’re in a big group and having a conversation with everyone else and then start it doesn’t work, but let’s say they were sat next to each other at a table and a conversation was happening one end and then the two of them just said a couple of words to each other I don’t see it being a problem?