How to use several languages with the built-in dictionary without changing the interface language?
On iOS7: the dictionary can be selected from the definition screen.

On iOS6:
From my experience, switching the keyboard language is enough to change the dictionary language. 

First you'll have to add the keyboards you need (this is a one-time operation): 


Go to Settings -> General -> Keyboard -> Keyboards -> Add New Keyboard. 
Select all the languages you're interested in.


Now you can use the key that is located to the left of the space bar in order to use a different keyboard (a list is displayed on long press).