My Thai wife spoke very little English when we first met, but after we got married she learned to speak, read, and write English entirely on her own. We were living in the USA, and she spent a lot of time watching TV (especially American football). She became a big fan of the Dallas Cowboys (which I don't care for, being a Packer fan and all), and quickly picked up on all the rules of the NFL.
She didn't have too many Thai friends, but became good friends with mostly English-speaking people. Today she can speak and read/write English as well as she speaks and read/writes Thai.
Bottom line; exposure to English-only communication.
Good luck.