This is very interesting because this reflects my own experience as a teacher in the ESL classroom. Often, some students who come from overseas to study in the US only see that living in the US is the key to improving their English; however, in some cases, they often do not make much progress because as soon as they leave my class, they pair up with other students from the same country and start speaking their native language again. The problem isn't that they are just associating with people from their country, but that they don't speak English when they do so.
Only the other hand, I have met many students who have made great progress in learning English because they are very motivated to do so and find every opportunity to study and put into practice their new skills. In fact, I know students who have learned English very well without having lived in an English-speaking country.
So, what has your experience been like? What are the factors that help students learn more quickly? Share your ideas.