One of the under-appreciated, but incredibly freaking awesome features of Street Fighter IV was the ability to not only toggle voice tracks from English to Japanese, but to choose which characters you wanted to adjust. Capcom knows what we like, and that's why they're bringing this feature over to Marvel vs. Capcom 3. Sort of.
While you'll be able to choose which of Capcom's brawlers you want to sound like goof balls in English, and which you want to sound much cooler in Japanese, Marvel's fighters don't appear to have that option. Understandable perhaps, but I would find much twisted pleasure in hearing Captain America scream out his Stars & Stripes in perfect Japanese.
Oh well. Never one to look a gift horse in the mouth, it's going to be nice to hear at least some Japanese amidst six fighters beating the X out of each other.