It is fairly easy. Kids are like sponges, soaking up everything without any effort or even concieus idea that they are in fact learning.
There are different scenarios:
The family is moving abroad for a job.
Parents are different nationalities and decide to teach their children their native languages.
Parents know a foreign language very well, so they speak it to their children at home.
Children are sent to school that the curriculum is in a foreign language, parents not necesarily have to speak it.
I personally know many families for every scenario. My cousin Patrycja, who lives in Poland talks to her son only in Spanish that she know very, very well. Our good friends, who lived in Toronto tough their kids Polish and French which are their mother tongues. The kids learned English outside, at school. In our case it was a job transfer to the USA where our children were born later.
We were sure that we wanted to teach our kids Polish, we just did not know how. We realized pretty quickly that it was not natural for us to speak to our daughter in English (despite knowing it very well) and it did not make any sense to introduce it at home since the whole world around us spoke it.
Nadia at a very young age realized that something weird was going on. Many times she had a surprised face, asking why everyone else is speaking different then my parents and I can not understand any of it:-) It changed very quickly, playing with American children every day, she reached their language level in few months. The situation was finally clear - Polish at home, English with everyone else. Sometimes she tried to mix, but we repeatedly and consequently asked her not to.
When she went to school, we started to introduce at home writing, reading and Polish grammar.
At this point she can write, read, speak, understand both languages and she just turned 7 last month.
In the meantime, our son Alexander was born. With his language education, we applied the same approach, except it was much easier because his main teacher was... Nadia. Now, he is 3,5 years old and does pretty good in both languages.
Here in San Miguel de Allende our kids learn a third language: Spanish. We enrolled them in Spanish only curriculum school so it is complete immersion. It has been 4 weeks and they can have a simple conversation, buy groceries at a local store and order their own food at a restaurant. Pretty good as for one month of school...!!!! :-)
Nadia often finds similarities to the languages that she already knows. That only proves that it is easier to learn another language while you already know some.
We also decided to learn Spanish and we use Rosetta Stone program along with real life practice. I have to say it is pretty good and gives me the flexibility to learn at my own schedule. Most days we learn in the morning and in the afternoon we go out and try what we learned with the Mexicans who are very, very patient. :-)
What is your experience? Please share.....