Actually, no. Both the Greeks and Trojans spoke Ancient Greek, which means that English is just a substitute for the language. What I'm asking is why the Empire and the Republic speak the same language, when they both formed in almost complete isolation from each other, not why Galactic Basic Standard sounds like English.
If you play a Republic character, it might be the Imperial NPcs you talk to only talk Republic Basic when they talk to you.
And when you play an Imperial charcter, maybe the english is just the substitute language for whatever language they use, and the Republic NPCs you talk to either use that substitute as well, or your character switches to Republic Basic.
I doubt anyone wants to listen to a language they don't understand throughout the whole game. Plus they would either have to invent a language, or you'd have convos that repeat the same sentence again and again, like when you talk to some Twis and so on.
Of course you could also go play on a german or french server and activate english subtitles (if that's possible) to get the immersion.
EDIT: What I meant was downloading and playing the german/french version, not playing on such a server of course.