The folks at Gist have added a new third party iOS 8 keyboard for your consideration, called Slated. Admittedly, this area is starting to become crowded, but unlike the other offerings that aim to add colors or new input methods to your keyboard, it has a more serious task.
Slated has the ability to translate what you type into more than 80 languages, in real time. Simply pick a language, start typing, and watch the evolving translation appear below the text entry field. It also translates backwards, so you can participate in complete conversations.