In iOS 8 very much like its immediate predecessor, there are only two dictionaries pre-installed (Apple Dictionary and the American Dictionary) by default. If you want to add any other dictionary in accordance with your use, you will have to go through some hoops.
Apple doesn't stock your iPhone with dictionaries although it does let you add keyboards in various languages. Apple downloads the corresponding dictionary when you select the language while setting up the phone. After this, you have to add dictionaries on your own.
It barely takes a few steps to add as many dictionaries as you want.
Here's How to Add more Dictionaries in iOS 8
Step #1. Tap and hold on some word in an app like Notes, Safari etc. (any app where you can select a word.)
Step #2. Tap on Select/Define.
Step #3. Tap on Manage (bottom-left)
That’s it. The dictionary will be downloaded and added to the list of dictionaries on your iDevice.
How To Remove Extra Dictionary From iOS 8
If you don’t want a particular dictionary that you added, here's how to do that:
Step #1. Tap/Press on the word
Step #2. Tap on Select/Define
Step #3. Tap on Manage
Step #4. Tap on the X Button to remove the dictionary you don’t need
You are done.
While the dictionary will be “removed” from your iPhone/iPad, it's still available for a re-download if you choose to.
Apple added a few more dictionaries in iOS 8 (Russian, for instance).