Right after figuring out how to enable the built in Japanese to English dictionary in OSX a friend was like "Awesome! Now they just need it for iOS" to which I thought, hmm, I wonder.
A few clicks later and BAM! Same feature is in iOS
First select a word and pick "Define".
Now pick "Manage"
Turn on / Download the Japanese−English dictionary. (A small black circle apparently means it's on)
Next time you pick "define" you'll get a definition
Unfortunately you can't pick define on everything in iOS. Most apps don't support it and even browsers often don't let you select text, specially if the text is part of a link. But hey, it's better than nothing.