SwiftKey is admittedly a fantastic 3rd party keyboard for iOS 8. Like all 3rd party keyboards, it needs Full Access to the system. The actual effect of this is that the app can update its internal libraries (languages in this case) without the need to release a new app altogether.
Since SwiftKey needs to use Full Access because it’s a keyboard, the keyboard itself is separated from the container app you see on your device. This means the container app can look for additional languages using an internet connection, and language packs can be added any time without having to put the app through the review process.