My son is fully bilingual in Afrikaans(sort of a Dutch Dialect) and English. When he was a toddler he would speak fluently to his grandmother in Afrikaans and would turn in the same conversation and seamlessly talk to me or his mother in English. He kept this ability right into about the middle years in Elementary School. After that this ability somehow diminished and he would throw English words into an Afrikaans conversation or vice versa. He is thirteen now and speaks both languages very fluently (but the switching between the two languages now is more conscious). At the toddler stage it was as if his brain was combining the two languages into one.<p>From my own personal experience I noted that, there is a point when you start to think - in the new language and that is the real turning point. It also appears sort of in computer languages as well.<p>When people from different linguistic cultures mix they tend to develop their own language for example fanagalo [<a href="http://en.wikipedia.org/wiki/Fanagalo" rel="nofollow">http://en.wikipedia.org/wiki/Fanagalo</a>].<p>This paper from the Center for Applied Linguistics has some good pointers  <a href="http://www.cal.org/resources/Digest/digestglobal.html" rel="nofollow">http://www.cal.org/resources/Digest/digestglobal.html</a> as regards to languages and education.