I have a feeling that it won't have a Japanese option. Given the larger amount of characters with different voices(especially the Pokemon), the fact that there's three Star Fox characters who have sets of vocalized taunt conversations and Snake who gets several lines per opponent character, that'd alone be a lot of audio to have two languages of compared to Melee. Kirby Air Ride, which features NO voice work, not even for Kirby, didn't feature a Japanese option, so it's possible that after Melee that ability was removed. Brawl also has two layers, but the second houses (near?) full screen FMV videos... It's also possible they'd have to redo the audio in many of the videos, when they feature grunts and attack voices for characters with different voices. I'm guessing they wouldn't be able to fit all that comfortably.