Well... a lot of the bottleneck with kids is how much language their minds can handle at that point.
The analogy isn't perfect and it is open to interpretation. That said, watch your nephew develop over the next three years. Note that there is very little deliberate training or discipline involved. Keep in mind that he could do this for several languages in parallel.
By 5 or 6, his language skills will have progressed much faster than most adults' in a similar period. At 4-6, a few months of immersion (say summer with grandparents) will yield some mind-blowing progress. Again, not impossible for an adult... but rare in practice.
I grew up in a multilingual house. My father was an english speaker. We lived in Israel. He did speak the local language (hebrew), but I spoke it better by 6. I could also handle thick accents (irish) that my mother couldn't understand.
This isn't unusual. Kids translating for parents is a very common phenomenon for immigrant families. It happens because kids learn faster. Adults can make up the difference with discipline, but it's a tough contest.
I'll definitely keep a close watch over the next few years! They actually do have a somewhat multi-lingual household already, so he speaks kind of a mix: English for most things, Japanese for words such as breakfast or lunch.
I've been wondering if it would actually hamper his skills to have to learn more than one language at a time, but your experience at least seems to say the opposite.
Kids vary. The median ability seems to be 3 languages.
That said, I think there are hidden abilities. My younger brother would only speak one language. He understood both, and we both had a smattering of other languages (russian, arabic, polish, tunisian) too.
Seemingly, he wasn't bilingual (2-4yo). But, after a week of immersion (grandparents) he would fully switch language and stop speaking the first one. The ability was there, but he didn't like switching in real time.
If Your nephews get a week or two in Japan, they'll probably unlock another level of language ability very quickly.
The bonus is accent. On this, even a disciplined and talented adult can rarely come near children's innate ability. Accents are usually carried by adults decades after fluency is achieved. Kids don't have this problem. Again, immigrant families give us a wide sample group to observe.
The analogy isn't perfect and it is open to interpretation. That said, watch your nephew develop over the next three years. Note that there is very little deliberate training or discipline involved. Keep in mind that he could do this for several languages in parallel.
By 5 or 6, his language skills will have progressed much faster than most adults' in a similar period. At 4-6, a few months of immersion (say summer with grandparents) will yield some mind-blowing progress. Again, not impossible for an adult... but rare in practice.
I grew up in a multilingual house. My father was an english speaker. We lived in Israel. He did speak the local language (hebrew), but I spoke it better by 6. I could also handle thick accents (irish) that my mother couldn't understand.
This isn't unusual. Kids translating for parents is a very common phenomenon for immigrant families. It happens because kids learn faster. Adults can make up the difference with discipline, but it's a tough contest.