I can start from the title of an article which was published many years ago, in 1989, by Francois Grosjean.  The title of this article was, “Neurolinguists, beware! The bilingual is not two monolinguals in one person”. Grosjean meant to challenge the idea that a bilingual person is not simply the sum of two monolinguals. The experience of learning and using a second, third or even fourth language changes the whole language system in our mind. And this, not just in terms of quantity, that is, the amount of new knowledge and skills that we add to out former stock, but in terms of the quality of the whole system.