According to just about every science fiction movie ever made, the universe is amazingly diverse in life forms, wardrobe preferences and silly hairstyles (Princess Leia, I'm talking to you!). So why does everyone either speak or perfectly understand English, exactly? Oh yeah, because a lot of detail would fall through the cracks if audiences couldn't process what the heck everyone's saying.
Of course, Wookies, Klingons and the like maintain their own lingo/communicative grunts, but English-speakers still seem to understand with little to no effort. Those of us who struggled through standard high school Spanish are muy bitter and jealous, to say the least.