I've always tried to expose my kids to forign languages so that should they ever choose to learn one it will come easier, and learning a second language increases cognitive abilities. I can best rock babies to sleep while listening to Teresa Teng (Chinese) or Roller Coaster (Korean), and when my kids insist on watching the same cartoon over and over I used to sometimes switch the audio over to German. (Well, my daughter didn't mind non-English cartoons until she turned 5.)
Every now and then my kids will speak some gibberish, then tell me or their mom what it's supposed to mean in English. I think I've successfully planted the seed.
Surprisingly, when my son C-Ko was three years old he seemed to have incorporated some foreign words into his vocabulary. For a while he used to say “ん” in much the same way a Japanese person would, which in English can mean the affirmative “mhm” (or negative “m-m”; it's really subbtle in Japanese). It's probably just a coincidence, but for a while I could swear that he was also saying “어디 엄마?” which is Korean for “where's mom?” which I never taught him, and I don't think I've really exposed him to any Korean movies yet.
Three-year-olds aren't renown for their command of grammar, so it's not surprising that until he turned three he always used “me” as his first-person subject. He soon afterward finally figured out how to use “I,” but for a few months I think he was using it as the object and still using “me” as the subject.
Anyway, in English we have “my” and “mine” for the first-person posessive. Much of English has its origins in German, and the German first-person possessive is always “mein” (plus article endings). My son used to always say “mine” and never “my,” which isn't really much of a surpise since “Mine!” is one of the first things little kids learn to say, but I would jokingly tell people that he spoke German since he never said “my.”
He was about 3-and-a-half the first time I noticed him using “my,” and he appeared to be using “my/mine” correctly most of the time after that.
One night about that time as were were going to bed, my son said, “I have lots of Germans!”