The general rule of thumb is:-
High TL, sophisticated society:- your locals' accent is likely to be more highly refined, lots of fancy words, proper grammar even somewhat snobbish and florid. Think of Harvard, Eton, Henley. Think of how Poe and Lovecraft wrote, mostly adjectives, short on nouns and verbs.
Low TL, more rural society, garden world, the locals' vowels are going to get broader, speech somewhat coarser, fewer fully pronounced words, lots of contractions, use of grammatical mashups like "ain't" rather than "isn't." Instead of "I think that we should call the police," they'd say "Mebbe you ought run tell t'lawman." Rather than say "rather than," they say "'stead of."
Geeky, high TL worlds, the smarter locals are likely to use jargon, acronyms, buzzwords and regular in jokes: to an outsider, it sounds almost like they'd be talking in lolcat macros or something out of H P Lovecraft's Necronomicon.
Backwards former colony world, regressed a couple of TLs: Expect the language to contain a lot of archaisms, something like stumbling across a colony of people lost in time whose English comes straight out of Chaucer, with the resident medic peppering his speech with some Mediaeval Latin and Greek to make him sound "laarrned."
Even if everybody was speaking the same language, say Vilani, the accents would vary, possibly in extremis, depending on where the locals come from.
Think of the language used in
Firefly and the movie
Serenity.
Lowbrow:
Mal: "There's just an acre of you fellas, ain't there?" (to Zoe) "This is why we lost, you know. Superior numbers."
Zoe: "Thanks for the re-enactment, sir."
Zoe: "You sanguine about the kind of reception we're apt to receive on an Alliance ship, Cap'n?"
Mal: "Absolutely ... What's 'sanguine' mean?"
Zoe: " 'Sanguine'. Hopeful. Plus -- point of interest -- it also means 'bloody'."
Mal: "Well, that pretty much covers all the options, don't it?"
Mal: "Well, look at this! Appears we got here just in the nick of time. Whaddya suppose that makes us?"
Zoe: "Big damn heroes, sir."
Mal: "Ain't we just!"
Bandit #1: "And I think maybe you're gonna give me a little one-on-one time with the missus."
(Husband) Jayne: "Oh, I think you might wanna reconsider that last part. See, I married me a powerful ugly creature."
(Wife) Mal: "How can you say that? How can you shame me in front of new people?"
(Husband) Jayne: "If I could make you purtier, I would."
(Wife) Mal: "You are not the man I met a year ago." (they suddenly draw their guns on the bandits, Mal slowly pulling his bonnet off)
Mal: "Now think real hard. You been bird-dogging this township a while now. They wouldn't mind a corpse of you. Now you can luxuriate in a nice jail cell, but if your hand touches metal, I swear by my pretty floral bonnet: I will end you."
Mal: "You all wanna be looking very intently at your own belly buttons. I see a head start to rise, violence is going to ensue. Probably guessed we mean to be thieving here but what we're after is not yours. So, let's have no undue fussing."
Highbrow:
The Operative: You know, in certain older civilized cultures, when men failed as entirely as you have, they would throw themselves on their swords.
Dr. Mathias: Well, unfortunately, I forgot to bring a sword.
Dr. Mathias: [as the Operative pulls out his sword] I would put that down right now if I were you.
The Operative: Would you be killed in your sleep, like an ailing pet?
The Operative: [to Mal] You cannot make me angry.
Inara Serra: Please, spend an hour with him!
Dr. Caron: "These are just a few of the images we've recorded. And you can see, it wasn't what we thought. There's been no war here and no terraforming event. The environment is stable. It's the Pax. The G-23 Paxilon Hydrochlorate that we added to the air processors. It was supposed to calm the population, weed out aggression. Well, it works. The people here stopped fighting. And then they stopped everything else. They stopped going to work, they stopped breeding, talking, eating. There's 30 million people here, and they all just let themselves die."
Now in the not so exciting world that we laughingly call "real," think of the difference between the humour of Peter Kay, Jimmy Carr and Stephen Fry.
As a Referee, you don't have to do everybody's accents. In fact, I'd actively discourage it as a bad idea, drawing from my own disastrous experiences.
However, on occasion you might enjoy throwing in some sort of a situation where the ruffian who's just approached the characters is asking "Whassa teem?" with increasing agitation before one of them suddenly twigs and says "Oh, it's half past two."
