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.
When people from different linguistic cultures mix they tend to develop their own language for example fanagalo [http://en.wikipedia.org/wiki/Fanagalo].
This paper from the Center for Applied Linguistics has some good pointers http://www.cal.org/resources/Digest/digestglobal.html as regards to languages and education.