Funny thing is that I need to be in a mindset for a language.
A couple of years ago, I heard people talk in the train, and I had no idea what language it was. It wasn't German, it was no Scandinavian language. But, the rhythm of the words and sounds indicated somewhere to that direction. I did not understand a word.
Then something triggered me recognising it. The dialect my Mother has been raised in. And, I could understand everything they said. And, give me a day, I can speak it fluent again too. (Was fluent enough for people to wonder if it was my mother tongue or not)
If I have been thinking and talking in English for too long, I find it hard to talk Dutch. And, nowadays I use the spell-checker more for Dutch than for English, I hardly write in Dutch, and I make all kinds of English mistakes in my own language.