This is a question that pertains to the entire saga really, but my son and I were watching A New Hope this weekend when he turned and asked me this question:
(while watching the Han vs. Greedo scene in the Cantina)
"Dad, if aliens can understand English and humans can understand alien languages, why don't they talk to each other in just one of them?"
I sat for a moment letting that question sink in. My first reaction was, "Yeah, you're right!" Look at it this way, rarely if ever on Earth do two people converse back & forth using their own respective languages. They'll pick one and both use it for the duration of the exchange.
However in every case (I think) throughout the Star Wars saga (except for a couple of instances with C3PO), all alien/human conversations take place with two languages, English and the other.
My second reaction was "How astute of my son!" who is now thirteen to think of that. Considering everything else that I and others have discovered to be "wrong" with the movies, that one never occurred to me before. Interesting....