The problem with this is that you'd need to integrate.
There are a lot of immigrants living on my road. I only really speak to my NDNs on both sides.
One side - both young, both working and both I'd describe as fluent. On the other, the husband works and is fluent. His wife doesn't work, only really engages with her friends and her standard of Englush is very low. We exchange pleasantries - how are you? We are going on holiday. Where are you going? level of conversation. But she wouldn't have the language skills to work (they have young children).
I have friends who are second generation immigrants and their dads are fluent (I've spoken with them). Their mums came here 40 years ago and still can't get by in English because they've only ever really communicated in their native language with friends who have a shared language. Their husbands and adult children act as their interpreters.
You are right that if people immerse themselves in a language they will learn it quickly but, where they don't, just being in a country doesn't guarantee you'll learn the language.
Again, look at the British ex pats living in Spain who only want to eat at English restaurants, shop in English shops and don't learn Spanish because they have their own English speaking communities. They're not fluent in Spanish because they don't need to be.
So to become fluent, people need to work and/or have the basic level of language skills to enable them to engage and build on those.
A lot of people can get by with very rudimentary language skills and gesture but this doesnt equate to functional language which is all 'A Level standard' would provide.