Maybe you know this already, but I suggest you to investigate an app called Jspeak.
Take in account it requires a SIM in your device (so it would work on a smartphone but not in a plain tablet).
I've seen it used (and used it myself) in Japan and it was pretty good. Also, it was used by an old guy in a Ryokan in rural Japan. He didn't speak English so we just went to and fro speaking in his tablet, checking that the audio was correctly recognized (it converts it to text before translating) and then handling it to the other person who would read/hear the sentence in his own language.
I've seen it used (and used it myself) in Japan and it was pretty good. Also, it was used by an old guy in a Ryokan in rural Japan. He didn't speak English so we just went to and fro speaking in his tablet, checking that the audio was correctly recognized (it converts it to text before translating) and then handling it to the other person who would read/hear the sentence in his own language.