Hidden in plain sight within iOS 7.x's built-in dictionary is the ability to add a few more dictionaries. So while you think you're limited to U.S. English (or U.K, if you live in Europe), let's add some more dictionaries to the “Define” feature on your iPhone or iPad.
A dictionary is preselected for you when you select a language (when you setup the iPhone/iPad) but it doesn't mean you can't change it. Also, it doesn't change when you change the Keyboard language (Settings → General → Keyboard → Add Keyboard).
In iOS, you can “define” words to get their meaning from the built-in dictionary. The Define function is part of the context-menu (Cut, Copy, Paste) that shows up when you select a word.
You can select words in the Notepad, in Safari, from the text-field in Messages and more. When you select a word, you see this context-menu. Tapping the right arrow gives more options of which Define is one.
Tap Define and a dictionary opens up with the meaning of the word you just selected.
Do you see the Manage button at the bottom left? Tap on it and you see a list of dictionaries that are available for download. As of iOS 7.1, Apple offers 12 dictionaries besides the two already in it (Apple Dictionary and English).
The dictionaries include British English, Italian, Spanish, German, French, Dutch, Korean, Korean-English, Simplified Chinese (2) and Simplified Japanese (2).
To download a new dictionary from the given list, just tap on the cloud icon and the dictionary will start to download.
Prior to iOS 7.1 and 7.0, the way you added dictionaries was handled differently. The new method is obviously much simpler although a little hidden.
Also check out how to get different keyboard layouts on iOS 7.