I think it's pretty common actually among some immigrant groups (2nd gen Indians in the US at least, of which I am one). I was born in the US, but my first lan

I think it's pretty common actually among some immigrant groups (2nd gen Indians in the US at least, of which I am one). I was born in the US, but my first language was Marathi, and it was really the only language I was fluent in until I was around 4. After learning English in school, I started to always answer my parents in English. We didn't really live near a Marathi community, so it came to be that I could understand Marathi fluently (albeit with very limited vocabulary) around family but couldn't form a sentence to save my life. Same story for all my US/Canadian cousins, and most other 2nd gen desis I've met.