MC-820 is fixed during 1.6 development stage, but mob's message is still not translated.
According to https://gist.github.com/Dinnerbone/5631634, only base messages are translatable. Mobs and items are still only in English.

On the screenshot, base message is written in Japanese, but 'Player'(name for a non-premium account) and 'Zombie' is in English.