My friend asked me what my first language was. I grew up speaking English and Gaelic. I grew up in a moderately Gaelic community (about 30-40%). My dad spoke Gaelic and my mum English (she is an Englisher by birth). I went to a Gaelic school.
I would say I have two first languages. My English is now better than my Gaelic because I rarely get chance to speak it in London. But give me an hour and I’m back to normal.