Worked at a US company in Belgium where the lingua franca was English.
Moved to the USA 8 years ago.
My wife (from the Netherlands, but a native Flemish speaker) and I switched to speaking English at home.
Couple of weeks ago I'm walking in our neighborhood and I met this guy with a Flemish sounding name. He was Flemish. Started to speak Flemish to me. I could not reply. Could not find words, and didn't know how to move my tongue to produce those sounds.
Now, I can speak Flemish fine with my mom and sister. I struggle with words sometimes and my word order is incorrect.
But it only works when I call them. A spontaneous need to speak Flemish just... fails.
And forgive me for writing Flemish and not Dutch. It's different enough that it requires more active listening when you don't have a trained ear for it.