Maybe it's a nueral interface between your brain and the muscles controlling your vocal chords and mouth. You think a word in your native laguage, and it's gets translated before you actually speak it. The translator takes over and makes your mouth move in the correct fashion. The listener cannot tell a difference, because from their perspective their is no difference.
Of course this would not account for differences in biology. An alien species could have vocal chords that are capable of tomes that human one's are not. I guess then the implant could actually generate some tones and the facial movements will just be for show.
Of course it could be that all the actors at Paramount speak english.
Geoschmo