Little Kids and Language Learning

Even though I spent my early years in a bilingual household-- my grandparents, aunt, father and their friends all spoke Greek and English-- I lost my connection to the Greek language when we moved from the duplex my family shared with my grandparents to our own single-family home in New Jersey just before my 5th birthday. While I may still know a few words here and there, I am by no means fluent. I don't know what it's like to know two languages, let alone use them interchangeably.

Which is why I'm fascinated by my boyfriend's 3-year-old sister (aside from being one of the coolest little kids I know). My boyfriend and his family are Brazilian-- not of Brazilian descent, but actually from Brazil. His sister, though, was born and raised in America. Dear BF is completely fluent in English (even more so than some English-speaking people I know I'd say), and his mother speaks enough to get by, though she is more comfortable with and prefers to speak in Portuguese. His little sister can easily switch between English and Portuguese. 

What amazes me about the little peanut is that she knows when to use which language. She can say something in Portuguese to her mom, and then turn her head and speak to me in English. She'll speak in whichever she feels like to my BF, sometimes jumping between languages from sentence to sentence. Clearly, she is able not just to speak both languages, but to think in both, as well. Dear BF can do this too, and I think he's awesome. His ability to switch spoken and thought languages instantly absolutely amazes me. On the other hand, I somewhat expect an adult to know in what context a particular language is appropriate. Most adults can identify others as able to speak to English or another language. However, I thought that would be beyond the comprehension of a 3 year old.

It leaves me with many questions for those who grew up multilingual or have taught their children to be multilingual: 

• How do you learn two languages at once?

• At what point do you start to think in another language?

• How do you learn to differentiate between multiple languages as you learn them, knowing that you've learned a word in one language versus another?

I'm interested in learning other languages, and I'm wondering what I can learn from little kids about acquiring new vocabularies and their cultural contexts. I bet it would also be useful to building better ways to help first-year writers, who often struggle to acquire disciplinary vocabulary and "general writing skills"/academic language simultaneously.