My DD is finishing 1st grade at an immersion school with Mandarin, Japanese and Spanish tracks. She is in the Mandarin track and started when she was three. Her school uses the International Baccalaureate curriculum. It is student-led and question-driven, which I really like. There was a lot of focus on developing social skills in the preschool years. Without a great curriculum, I wouldn't like her school as much as I do. The language immersion is great and it's amazing to watch your kid pick it up so naturally, but it needs to be supported by a methodology that meets your child's needs.
As far as English, they just started with formal instruction in 1st grade (1 hour per day). My daughter knew how to read before starting 1st grade, but the formal phonics, spelling and writing instruction has been really valuable and I wouldn't have been able to provide her with those skills. I would be wary of a program that expected you to teach your own child English language arts.