My best friends sister actually wrote her masters degree paper about this subject, about teaching toddlers a second language. And to make matters even more interesting she tested it out, on her own baby girl. She started the test while the kid was 13 months old. She would speak to her in her native language and English as well. Its showing amazing results nowadays. She long done with her masters degree but she didn't stop with it, and the girl is now 2 and a half or so and she speaks both languages amazingly good, to the level where she sometimes doesn't even know that shes speaking two languages since its so natural for her now. Is sort of funny to hear her talk and i noticed a lot of people that don't know the background story just get confused when they hear her talk.