Is it a language you speak?
I now have an 8th grader & 4th grader from FCPS immersion. We don't speak the target language.
We hired tutors, normally au pairs from the neighborhood to work with our kids over the summer & breaks for just a little bit. I'd also toss in a week of language camp if it fit the summer schedule. We also got an international DVD player for watching TV in the target language. Don't know if you could stream stuff now. But, I found watching TV to be very "natural"
We didn't worry about it too much. It kinda fits with what you do on the other subjects - ie make sure they read over the summer, have them figure out the change when buying ice cream.