For whatever reason, my iBooks dictionary is stuck in Japanese. Even when I read English books, defining a word always causes the app to use the Japanese dictionary. My iPod's language is English, so I have no idea why this might be. Is there any way to switch it to the English dictionary?
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.
Open the Dictionary app, then Preferences, then choose the dictionaries you want. You can even choose the languages to use when searching Wikipedia.