No, it's not possible yet, only English and Japanese as provided by Apple. Extra dictionaries are available from 3rd parties for the OS X version of Dictionary.app, but Apple does not provide any way to add these in the iOS version so far.
The link is broken - please look at some upvoted answers below as well
Assuming you are talking about the reference dictionary for looking up the meanings of words (NOT spellchecking/autocorrection), you cannot modify that on an iOS device. But in OS X you can add other modules. This page provides some info:
Best Answer
There is a freeware, Language Switcher, to launch an app in a different language.
Or, you check another thread for more detail: Is it possible to launch a program in a specific language?