In (non-Quebec) Canada, there's often an option to do French Immersion, even though the primary language is English. My parents put me in this program for elementary (K-8) school, and gave me the choice for high school, which I opted out of. There's a few positives and negatives that I found watching myself and my peers:

Pros:

- The 9 years that I spoke French in the classroom for every class except for English was enough to make the language stick. I'm 31 now, and I can still speak, read, and write French. My vocabulary is weak since I don't use French on a regular basis, but when I was in Paris, I had no problem understanding what was going on, ordering food, taking a cab, etc.

- I feel like having the 2nd language has made it easier to pick up other languages. I'm by no means fluent in Spanish, but when I was regularly doing DuoLingo Spanish lessons, I felt like things stuck very quickly.

- I also feel like some kind of structures in my brain from this help me pick up different programming languages. It seems like, compared to most of my peers, switching between new languages and learning new languages is generally a straightforward task for me, including weird ones like Erlang and Haskell. The majority of the effort goes into learning the concepts, not learning the syntax.

Cons:

- After graduating high school doing full immersion all the way through, many of my peers struggled with writing in University. I distinctly remember my first roommate getting an essay back and he was freaking out about the terrible mark he got. He asked me to look over it to see if I could see what was wrong with it. I read through it and just told him "Well... the problem is that you used English words, but you wrote it in French."

- Likewise, many of them had difficulties in math and science, where they understood the concepts very well from high school, but didn't have the English vocabulary to describe what they were doing.

All in all, I'm super happy that my parents made the choices they did, and that I dropped out of immersion before high school. I think, too, it helped a lot that I read a pile of English books growing up, both fiction and non-fiction, which hopefully helped supplement my English vocabulary while I was learning French at school every day. I was the 10 year old who held on tightly to his copy of the Peter Norton pink shirt book :)