In my city, Japanese, Chinese, Russian, German, and Spanish are taught in public elementary schools. There are only so many spots in each program and they are chosen by lottery. My kids are in the Japanese immersion program, one does really well, and the other struggles. I'm not sure about the other programs, but the Japanese program goes all the way through high school. In elementary they spend half their day learning in Japanese, but middle and high school it goes down to one or two classes a day.
And most of our schools have ESL teachers, too. There's a very diverse population here. I think another way to help adults would be to volunteer at a literacy program. My husband had looked into it and a lot of the students were immigrants.