I think making the device is the easy part, the hard part is doing the translation right in most of the cases, which both Google and Bing have been unable to do so far. 'Google Translate' app already has a communication mode where two people with different languages can talk, but if it worked good there was nothing preventing them to make such an earpiece.
It's certainly far ahead of having no language, or trying to look up a phrase book.
BTW It's like "word lens" for audio https://en.m.wikipedia.org/wiki/Word_Lens Google bought and made it free. I've tried it, and it works, it's very cool. But I don't know if it's actually that useful, or even gets used much, in the field.
[] EDIT sorry, those figures were just for speech recognition!