As a kid I had a Norwegian school friend ... at home they spoke Norwegian ... very harsh language to my ears. Sounded like they were always pissed off. Maybe they were ... :( ?
My dad was born in Norway but came to the US when he was 7 or 8. He came not speaking any English, but his mother and stepfather could (long story but his widowed mother left him behind with her parents when he was a toddler and went back to get him after she had remarried and established here).
My dad told me he was enrolled in school, but a grade behind other kids of his age, and was bullied because he couldn't speak English and was bigger than the other kids, so they thought he was "slow", so he, by emersion and necessity, learned quickly.
His parents spoke a mixture of English and Norwegian at home.
But I remember when my Norwegian grandparents would come to visit us, my grandmother who by know spoke perfect English, would start taking to my father in Norwegian.
My mother who didn't know any Norwegian suspected her mother was trash talking about her and she was right.
My dad never spoke Norwegian at home as I was growing up, except for a few words and phrases and I wish he would have.
Kids who learn a second language when very young have a much easier time learning other languages, even unrelated languages.
FWIW, my dad did have a bit of an accent, but it was Northern New Jersey.