Microsoft Translator is the company's cloud-based translation service, primarily geared towards business use. With the latest update, version 3.0.183, the Android app now supports Japanese voice translation.

As with any of the supported languages, users can simply speak into the app to translate to/from Japanese. Microsoft Translate even has a conversation mode, where each person can hold down on a microphone and the phone will read out the translation in real-time.

I can't speak Japanese, so I can't vouch for the accuracy of the translation, but it's pretty neat regardless. You can find a link to the app below.

Translator
Translator
Price: Free