Most of the time, it has to be all Japanese, or none at all.
Consider it this way:
Whenever a Sub is made, it's almost never acknowledged how everybody understands each other (even if they are from different countries) in some anime (or how they can all speak Japanese). They never explain if they're actually using another language (and we don't know about that), or if everybody is some kind of perfect w€@b00 (no offence).
Dubs are only good if they portray a country and characters outside of Japan, but they never make the Japanese characters in them sound like they're supposed to (am I suppose to assume that they get those perfect American accents from studying in New York or something?).
Anyway, my point is that it would be much smarter to combine Subs and Dubs in some cases to acknowledge the logically realistic language barriers between some characters (nowhere is this more apparent than Jojo Stardust Crusaders, as one of many examples, or Black Lagoon, where a dub makes sense, except for the part where the one Japanese member talks like an American apparently), and I rarely ever see it properly played out.
I think a fantastic example is what they did in Kite (you know, Quentin Tarantino's favourite Hentai) where the twins she has to kill later (and their bodyguards) are voiced by actual Americans (with Japanese Subs, in a twist of Irony), or Christine Robbins from Itazura na Kiss (along with all of the tourist scenes).
Does anyone else share my dumb sentiments?
