Spoke Dutch only until kindergarten...learned English "on the fly" in school. I can remember some of the strange looks from the teachers!
When I was very young, I spoke Dutch but understood when people talked to me in English. Then I started speaking English, but still understood Dutch relatives who spoke to me, although I'd started losing the ability.
In high school and college I studied German for years, mainly due to the fact that Dutch and German are quite similar, so it was an easy "A" for me. I already had most of the grammar and syntax down! But all those years of studying German basically destroyed what little understanding of Dutch I had left!
PS...I also remember being very jealous, watching my parents and older brother get sworn in as US citizens. They got little American flags, and I didn't!! [I was the first in my family born in the US, so I was already a citizen.]
Len