From people who have been put the through the Language Immersion Courses it helps greatly in learning the foreign language including particular dialects if needed.
From my dealing with recent immigrants in the US I've seen them retain a heavy accent mostly because the person uses the native language just as much if not more then the language of the new country (English in this case).
So I would have thought that an English Immersion program would be beneficial but I was never able to find any in the US.
Are there such programs publicly available?