Google Translate is the gold standard of all mobile translation apps, offering access to 65 languages (including the esoteric Lao Thai dialect). It works exactly like the Google Translate Web tool, except it's been shrunk to fit right inside your iPhone or Android device.
For 17 languages, translate text just by speaking into the app -- great for haggling at densely packed market stalls in Hong Kong. For 24 languages, listen to your translations spoken aloud. Or, after your translation is processed, tilt your phone sideways to display the text in full-screen mode. During our hands-on experience, we were thoroughly impressed by the app's almost-human speaking voice. Google even got tiny nuances right, including sounding out different tones when we fed it English-to-Chinese phrases.
Insist on learning Chinese phonetics? The app can spell out the translation of character-based languages in Latin characters for you. We asked Google Translate, "What is the weather today?" and it spit back both Chinese characters and the pinyin representation of the sentence. Handy tools also allow starring of common phrases for quick recall and access to translation history, even when you're offline. We liked that for those in truly multilingual settings, Google Translate included dialect preferences and even a detect language mode, translating our input to whatever end language we chose.
The Android version of Google Translate even adds handwriting recognition for input and augmented reality to translate whatever you see, whether it's a road sign or a block of printed text from a book. When you're done taking a picture of the text, use your finger to highlight the portion that you'd like to convert into English.
We found this augmented reality feature mostly accurate, though there was the odd instance where character recognition didn't work completely (one time the app detected "object" instead of the word "subject," for instance). It's worth noting that Google Translate typically worked better for signs than with text documents.
The Google Translate app has everything you need: dictation, voice recognition, augmented reality (on Android devices) and the deepest language database we've seen. If there's one app that you add to your arsenal for translation purposes, Google Translate should be it.