My interpretation of Professor Suzanne Flynn's research into second language learning is that the structures of our first language remain imprinted in our brain, and new language learning is filtered through them. The evidence comes from analysis of errors made by learners of different languages in learning English, which reflect the structures of each learner's own first language.
Here is a snippet of confirmation. My piano teacher is French, and our lessons are in French. He tells me to play a Si. I say I'm already playing a C. Then I realise that his Si is our B, and play a B. He has the same trouble moving from note names in French and English. (He is, incidentally, a most distinguished pianist, and I'm lucky to have these lessons. Check him out if you like piano music - Patrick Hemmerle). The morale is that we never lose our mother tongue - and that it never ceases to operate, whether we speak our first or second language.