Bing Translator is a great, free app for Windows 8 available in the Windows Store.
Here is a quick video I made, showing the main features of Bing Translator:
You can translate by typing text, taking a photo of some text, or by speaking directly into your tablet or computer.
You can use the app by itself or you can translate from any app using the Share Charm and sharing to Bing Translator. It also works very well in snap view so you can translate quickly while doing other tasks.
If you pin Bing Translator to your Start Screen you can improve your vocabulary with a ‘Word of the Day’ on the Live Tile.
You can translate between over 40 languages.