Do you have to be born into a language to be fluent?

So I recently finished college with a degree in Japanese. So I’ve been in classes studying that language for 8 years. I also lived in Japan for a year working as an English teaching assistant. I would say I’m pretty proficient in Japanese. <br><br>The thing I’ve noticed about other people who are proficient/fluent in a language is they’re usually fluent in 2. For example, a Hispanic person is usually fluent in both English and Spanish. <br><br>What I mean is if you are born into a language, you don’t necessarily have to grow up in a household that speaks that language. So if you grew up in England but born to Indian parents and you learned both languages as a child, in my opinion you will always understand that language more than someone who is trying to learn it as an adult.<br><br>So in my case, I’m not fluent in Japanese. I can speak it well and understand most of what’s being said but Japan has a lot of context that I won’t understand unless someone explains it to me.<br><br>So I guess my question is, do you have to be born into a language to actually be fluent? Or can you be fluent in a language that you weren’t born into?<br><br>Edit: thanks for all the responses so far!