One of my favorite jailbreak tweaks for the iPhone 4S (before I updated to iOS 5.1 and lost my jailbreak) was called Lingual, which enabled Siri to connect to Microsoft’s Bing Translate API and immediately translate words or phrases into a number of different languages.
Thanks to iTranslate Voice, you can now get the same functionality and more for any iPhone from the App Store. This excellent $0.99 app listens to you speak, then quickly translates your sentences into one of 31 languages. It’s super quick and incredibly accurate.