Questions about nationalities and languages in Death Note
At least for L, his official profile says that he's fluent in Japanese, Chinese, English, German, French and Russian. I wouldn't be surprised if the other high-IQ guys were multilingual as well.
The author says L is quarter Japanese, and since he has black hair and black eyes, introducing L as Japanese shouldn't be that strange, I guess.
Again, at least for L, his profile says he's British (イギリス国籍).
Dunno.