Language learning is different for many different people. I think developing different strategies for different learning styles is an important thing to think about. Some people learn better while they're at work. Having programs where language can be learned in a work setting is one good practice I've seen and I think should be considered. But, yes, it's certainly a challenge.
The other thing that I think volunteers and sponsors bring is a chance to interact with people who speak English or French in their community as a first language more often. That interaction, I think, can bolster anything that's in a classroom.