Woah. His reaction to her speaking Korean to him the first time is surprising! And what a shock for her, too, to suddenly feel like she had alienated her baby. Ouch.
The linguistic development in children is a fascinating thing... I've known several multicultural children who were fluent in several languages by the time they started school.
I also had a girlfriend who had learned Czech when she was a child, and then got a job in Prague. And the first day at work she was mortified to learn that she had basically learned "baby talk" in Czech, and couldn't carry on a business conversation. She had to do a crash course in "adult Czech" to be able to do her job!
And for me... anytime anyone speaks to me in any foreign language, I immediately respond in Hungarian... the only foreign language I was ever any good at. That's a weird brain-thing; I hear a foreign language, so I automatically speak the only one I know... a small, niche tongue, that is rarely heard outside its home country, and has little use in the world.
At least I know the "formal" form, so I can speak to Hungarian grandmothers politely. Because THAT happens so often... not. :-D
Yes! As someone that is fluent in 3 languages, I find the concept of "mother tongue" especially fascinating. My russian is pretty good, but it was essentially shaped by my parents, their friends and Sunday school in Canada. I remember going to Russia when I was 18, and everyone being amazed by the fact I was born in Canada and had no accent. But at the same time, I felt like my slang was stuck 20 years back in time and speaking to people my age was especially weird since I didn't know how to swear non-awkwardly and we didn't use the same filler words like "like". And the thing you'd do in a casual conversation where the Russian word doesn't come quickly enough in your brain so you just say it in French and keep talking knowing everyone will understand you doesn't work anymore.
I even stumble on the same phenomenon in Toronto where people give you weird looks when you sprinkle in French words par ci par là. I find it funny when it happens, because I immediately feel like a spy that has been caught, and often people ask you where you're from right after. In Montreal, even the anglos understand what you mean when you do that because of context cues and that's just the way we talk.
Since I went to an international high school, my french is also very "french from france" at times, and I have never had many close "Quebecois" friends until recently. And sometimes the words they use really take me by surprise and I love it how they have such a mastery over their mother tongue. And sometimes I feel a bit jealous I don't have such a mastery in any tongue. Finding my blank spots is a fun exercise - for example I realized only this weekend I really only know wild mushroom names in Russian. So when I'd find one i thought i'd recognise, i'd show it to my boyfriend and he'd confirm my IDing. But we could not say what it was to any of our friends.