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
Finland’s current president Alexander Stubb’s family language is Swedish even though I’m sure that his wife and kids speaks English better. His wife is from Scotland. Alexander’s dad is Swedish speaking and mom was a Finnish speaker. Alexander himself speaks Finnish better than Swedish (after elected he forgot some Swedish words and had to ask the press was it correct). Wife understands Finnish pretty well but isn’t fluent speaker. Swedish is so much easier to learn than Finnish so I guess they decided that hey we roll with that. (edit. and if you didn’t know, Finland’s 2nd official language is Swedish. Also Alexander is a polyglot. He speaks Finnish, Swedish, English, French, German and Italian).