[quote=Wayne]
I wonder if there's been animes in which there's both English-speaking characters and Japanese-speaking ones, but instead of having different voice actors so that all of them are speaking one language at a time, the linguistic division is clear within the show's characters themselves and there's only subtitles for the characters depending on whether it's in Japan or English speaking countries. Japanese subtitles for English-speaking characters in the Japanese airings and English subtitles for Japanese-speaking characters in the American airings.
[/quote]

It's a cool concept, but that'd be a lot of money thrown at voice actors to cover 2 languages like that.
You'd also need to explain why a conversation is going on in two different languages.