Such people are sometimes called Grenzemensch (border person). They grow up speaking multiple languages and don’t even realize til they’re older that the languages are different. They just think you have to talk to Uncle Fritz one way and Grandma Mireille a different way.
When you can speak and think in the new language without translating to your native language in your head, maybe? That can actually happen pretty fast. You don’t have to become anywhere near fluent.