i know of some bi/multilingual families in the us who’d talk to each other in their native tongue when they didn’t want the kids to know what they were saying.
i speak my dad’s native spanish as well as dad’s learned portuguese, but i don’t speak the polish or norwegian from the other side of mom’s family. (she’s also latina but doesn’t natively speak spanish)
however, i’m learning the two i don’t know, and practicing polish (the language my mom does know) with her
Both of my parents can speak our native language fluently but I can’t. I can say some words and understand it slightly but I was never taught I’m nowhere near fluent.