For what it’s worth, usually (but not always) the Imperials in Star Wars speak with English accents, and the Rebels (and people from the fringes of the galaxy) speak with American accents. In the original film, Leia uses an English accent in the early scenes, when she’s talking with Vader and Tarkin (and acting as a Senator); once she’s rescued by Luke and Han, that accent is gone. If we assume that the disappearing accent wasn’t a mistake, then it implies that Leia “code-switches” between how she speaks with Imperials and the government, and with the Rebels.
Also, in the Clone Wars cartoon, it was canonically established that most Twi’leks speak with French accents.
At the risk of becoming that guy, it should be obvious that “along time ago in a galaxy far, far (extra far!) away” aren’t humans. Using humans for the main characters is just shorthand. The dominant species are cast with humans, because it gives the viewer an idea of the power and racial dynamics ("We don’t serve their kind! " and where is Chewbacca’s medal, (aliengod)dammit!). They speak English because no one is going to read subtitles. Initially there were even English words on some displays, and Arabic numerals, but Lucas retconned that away.
Vernor Vinge incorporates this idea brilliantly into the novel ‘A Deepness in the Sky’. Everything is explained and described in a way that makes sense to the reader but it is made clear, for clever plot related reasons, that reality is far stranger.
Jimmy Steart, the opening to 1950’s film Broken Arrow
Tom Jeffords: This is the story of a land, of the people who lived on it in the year 1870, and of a man whose name was Cochise. He was an Indian - leader of the Chiricahua Apache tribe. I was involved in the story and what I have to tell happened exactly as you’ll see it - the only change will be that when the Apaches speak, they will speak in our language. What took place is part of the history of Arizona and it began for me here where you see me riding.
One non-canon explanation I’ve seen in fan circles is time travel; they aren’t just in another galaxy, they lived “long ago”. And since they aren’t sure of their homeworld, their ancestors appearing thoguh some sort of time-traversing wormhole or the like then forgetting in the ensuring tens of thousands of years where they came from works perfectly well for, say, some fanfiction writer wanting to fuse Star Wars and another sci-fi franchise together.
I assume that is because at this point everyone’s heard the opening line of Star Wars over and over again to the point where it’s a cliche. I assume that in 1977 everyone would be, “Oh, it’s like that fairy tale opening, but with ‘galaxy’ instead of ‘kingdom’” because that’s the cliche they’d know.
The characters were space aliens from a distant galaxy. Lucas hired humans to portray space aliens. And had them speak English so the audience could understand the dialog
Well, you see, Melkor disrupted the Ainur’s song, so Eru Ilúvatar raised his hand to sound a new theme, and the second time he did that, Man was created. Or was that a different fantasy world?
Oddly enough, there are almost no unadorned humans; there is a common species which looks almost like humans wearing foundation, contouring and other television makeup, though.