Learning a language by Immersion
This is a part answer.  According to "Immersion Education for the Millennium:
What We Have Learned from 30 Years of
Research on Second Language Immersion", (Cummins, 1998), based on the Canadian example of French language immersion state:


  For immersion education to attain its maximum potential it must be integrated into an
  educational philosophy that goes beyond just the discipline of Applied Linguistics. Students
  must have opportunities to communicate powerfully in the target language if they are going
  to integrate their language and cognitive development with their growing personal identities.


I noticed this as well when I was a teacher in Japan (I am Australian), where (and this is an anecdotal observation) multilingual students seemed to pick up the language by immersion, rather than the classroom.  The same applied to me, in Japanese classes I did not do too well, but immersing myself into the surroundings, I was able to pick up the language (at least the functional parts) very quickly.

A great reference is "Integrating Language and Content in Immersion Classrooms:
Research Perspectives" (Swain, 1998), making an important point that the traditional way of teaching aspects of language, e.g. grammar in isolation, i.e. without a context is not a particularly effective means of language development.