In Real Life, even when people from different places technically speak the same basic language, they don't always all talk the same. People from different regions, ethnic groups, and socioeconomic strata have different pronunciations, rhythms, and slang terms and idioms. In some works of fiction, this isn't true; they adhere to the Planet of Hats linguistically. On the other hand, there's this trope, where a work creator has created variety within the languages used in The 'Verse, typically as a means of World Building.
Bob, from New York, speaks with an American accent. This contrasts him with Aerith, who comes from a magical alternate reality hidden inside of a wardrobe, who speaks with a British one, despite not actually being from Britain. All of the people from Aerith's world speak similarly to herself, with maybe some variation between the different kingdoms or social classes of her land. The point is, people from Aerith's world have linguistic quirks that set them apart from people in the Bob's world as a result of the distance between them. Though the quirks in this example are inexplicably borrowed from a real world place that supposedly also exists in the setting, the proper creative intention is there and it still counts as an example.
If Bob and Aerith are both from fictional places, say they're both from two different planets in a sci-fi setting totally disconnected from the real world, and these differences still apply, then that's also an example of the trope. If the author goes a step further and has them speaking in two different real world languages, or using two different real world writing systems, or even both speaking and writing in their own separate made up languages, that also applies. The point is that the author invents unique linguistic traits for a fictional group of people, whether they be recycled real world linguistic traits or made from scratch, to make them more realistically foreign to the audience and/or each other.
The creative decision to more realistically define a fictional culture through language, where such distinctions do not already inherently exist due to the culture not existing in the real world, or existing in the real world, but being far enough in the future that major changes in language are justified, is key to the trope. Localizing linguistic differences to characters based solely on personality or to more accurately depict real world locations and cultures is not this trope. If Aerith only sounds British to show that she's a genius, not because she's fictionally foreign, then that isn't the trope . If Bob is from modern New York and Aerith is from past France and they both sound appropriately different and accurate to what a person from those times and places should sound like (or they don't, but that was the intent) that's just regular old realism, not the trope. It has to be fictional cultures, but realistically utilized linguistics.
A Subtrope of Multicultural Alien Planet. Compare Aliens of London, another trope that deals with giving accents to fictional races. Contrast Smart People Speak The Queen's English, where an accent is used as a shortcut to show a character's personality rather than because of their nationality, Aliens Speaking English, where people from fictional populations from outside of Earth speak Earth languages without explanation, or Anime Accent Absence, a tendency in Japanese media to not bother giving foreign characters distinguished accents at all.
- The War of the Masters
- The dominant local language of Moab III is Viet, which is Vietnamese mixed with various human and alien tongues including Klingon and Hebrew. (It's usually rendered as Romanized Vietnamese due to Translation Convention.)
- Recurring character Captain Sandra Pickens has a thick Funetik Aksent (based on Appalachian West Virginia) due to her upbringing on the Earth colony Beaumonde.
- In "Remembrance of the Fallen", Kojami Sobaru notes Kanril Eleya, both of them Bajorans, to speak their mother language Bajor'la with Kendran accent. In Create Your Own Fate Eleya notes that Reshek Gaarra has a Dahkuri accent.
- Diane Duane developed a partial Romulan Conlang for her Rihannsu novels. In the final book of the series, Uhura remarks to Kirk that the dialect of the language spoken on the rebelling planet Artaleirh has diverged so far from the version spoken on Romulus that she doubts the warbird crews sent to crush the revolt will understand one word in ten.
- In The Wheel of Time:
- Several mainland nations have linguistic quirks common to their natives, like Illianers' liberal use of "do" and "do be" as auxiliary verbs and Taraboners' tendency to end sentences with a "...yes?"
- The Seanchan invaders have a highly distinctive slow, drawling accent, and several of them remark on how mainland accents are incomprehensibly fast and clipped by comparison.
- The Sharans speak in a Creepy Monotone when their army shows up out of nowhere to fight for the Shadow in the Final Battle.
- The Aiel are on Full-Name Basis with everyone: among themselves, they don't have surnames and consider it a very intimate gesture to use an abbreviated pet name.
- In X-Wing: Starfighters of Adumar, the eponymous Lost Colony speaks a dialect of Galactic Basic with slightly different pronunciation: "Rad Flat" is heard in an early scene, referring to Wedge Antilles's Red Flight. Later, he greets another officer, who responds with an accent described in Wedge's Internal Monologue as "clipped, precise, Imperial", and Wedge recognizes him as being from the Imperial Remnant. (The latter references the Star Wars films' use of Evil Brit.)
- In the Foundation series, the planets of the Galaxy have different dialects, sometimes barely intelligible to one another. Magnifico, for example, speaks with the accent of the galactic core, conveniently rendered as Flowery Elizabethan English.
- The English dub of the first game has two examples. Though all of the characters sport accents from across southern England, most of them are distributed based on character rather than nationality (despite living together for most of their lives, siblings Fiora and Dunban speak in Estuary and Received Pronunciation accents, respectively, for example). The first exception and example of this trope is the High Entia race, who all speak in Upper Received Pronunciation accents and are the sole users of it. As a very long lived and somewhat isolationist society of High Elfish people, the language difference checks out. The other example is the Nopon race, who aren't so different accent-wise, but speak in their own unique dialect full of broken English and strange terminologies.
- Xenoblade Chronicles X implies that the various alien races encountered on the planet Mira have their own languages, but an unexplained phenomenon on the planet translates them all, so we mostly end up getting everyone speaking the same language in the same way. Some quirks still manage to make their way through though, such as the Ma-Non tendencies to repeat conjunctions or phrase sentences as questions, or the many oddities of the ever present Nopon dialect.
- The English dub of Xenoblade Chronicles 2 kicks things up a notch by giving every one of the fictional nations of Alrest their own set of dialects taken from the real world. The Ardainians all sound Scottish, the Urayans are all Australian, the Gormotti are all Welsh, and so on. The Nopon return yet again sporting working class southern English accents, presumed to be the native dialect of the Nopon founded Merchant City of Argentum, as well as their unique broken speech patterns and terminologies.
- Dragon Quest VIII features many accents to liven up the various regions of the world, most European in origin. The most common one is the standard British accent used for King Trode, Jessica, Angelo and many NPC's. Other examples include the people of the snowy Orkustk region having Russian accents, Morrie having an Italian accent, people from Pickham such as Yangus and Red having Cockney accents, Ascanthan natives having a Yorkshire bent to them, People from Baccarat having American accents, and Dragovians having a Chinese accent to emphasize the distance their land has from the rest of the world among other accents held by notable NPC's.