How can I switch between the Japanese and English dictionaries on iBooks?
This answer is for iOS 7, maybe 6.

The question might be stale, but the ANSWER (if folks are on iOS7, maybe 6) is this:


Look up a word in iBooks, or in any App that has 'Define' flyouts from text. You'll get to the Definition UI which is in Japanese… if you can't read it go for the button on the lower left.
At the bottom of the definitions, you'll see 2 'buttons', from Left to Right: "Manage" and "Search Web". You should see this whenever you get UI to invoke iOS's Dictionary (which is not an App like it is on OS X)
Hit "Manage" and now you have a list of Dictionaries that you can enable/disable. AFAIK, enabling and disabling is global, so if you do it in iBooks it'd be the same in TweetBot, etal. 


To conclude: There's no direct access to the Dictionary.app in iOS, even in Settings.app. You have to define a word, and manage dictionaries from there.