Fast language learners have a greater volume of white brain matter, which helps to process sounds, and send them into the lower levels of the brain--according to a research by the University College London.
Also, their brains are likely to be less symmetrical than those of others.
Remember the news about musicians having more grey matter? Here they pointed out that " Repeated flexing of the brain by practising a musical instrument could account for the extra grey matter in the auditory cortex."
What I miss in this article on language skills vs. white matter is exactly the importance of practice potentially increasing the amount or size of the fibres concerned.