Star Wars is set a long time ago. So is Pride and Prejudice. English might already exist.
This is where we get into the whole Ringer-style pretention of
The Journal of the Whills being a notional source document for the saga we know....
Anyway, we know from multiple explanatory tomes (The New Essential Chronology, The Essential Atlas, The Essential Guide to Warfare) that humanity arose in the Core Worlds region and spread out through the galaxy, first in sleeper ships then in hyperdrive-equipped vessels. Cultural splits started happening amongst prominent worlds, most notably Alsakan (Great Houses with titles, use of the High Galactic (Latin) alphabet) and Coruscant (more corporate-friendly, use of Aurebesh). Later on, Corellia (which had a royal house prior to about 800 years before the films, then became a republic) developed its own sphere of influence around two major hyperlane extending from its system, antispinwards of the previous expansions. Eventually the Seventeenth and Last Alsakan Conflict (approximately 3000 years before the Great Resynchronization -- for reference, the Battle of Yavin was in 35 GR) was settled at sword point in the Galactic Senate offices by the Corellian monarch of the day.
My concept that is mine (and seems to be born out by the films), is that while Coruscant and Alsakan had different writing systems, worlds in their spheres of influence had a similar sound to their Basic dialects (ranging from Received Pronunciation English to the sort of wandering Mid-Atlantic we here from Carrie Fisher and Natalie Portman). Worlds in the Corellian sphere of influence (Tatooine became a
de facto Huttese dependency, but is very close to to the Corellian Run) tend to sound more like American English to our ears.
Of course, this doesn't factor in regional enclaves such as the Tapani Cluster, the Atrisian Commonwealth or the Tion Hegemony -- but to the best of my knowledge we haven't seen or heard anyone from those places either, so a future writer has some room to plant some ideas.