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.
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.