@mangeurdenuage From what I recall of your life story, spending your time working at your parents' business instead of interacting with other kids (and gradually learning to speak the language again) probably contributed. If you were younger when your family returned to France, you probably could have even gained a native accent.

I lived on a military base in Germany for a few years growing up. But because we were mostly on-base with other US families, I barely learned any German ... and more than fifty years later, I don't even remember any of it.