*Slaps forehead*
Jesus Christ, I realize we're all nerds here, but...why are you over-thinking this so much?
Simple explanation: He goes back to speaking in a "Russian" accent so that the movie's audience can tell that he's actually Russian. Movies like "Salt" are made for dumb teenagers who think that being a sleeper agent means that the bad guy has to speak with an accent.
Duh. No essay-length post necessary.
