In Real Life, humanity has thousands of living languages spread across the world, due to a wide variety of cultures. In Speculative Fiction, however, all inhabitants of a planet have the tendency to speak only one language.
In Science Fiction settings, when an alien race is introduced and they aren't speaking English, their entire species apparently only speaks one language. Bonus points if the language is named after the race. This doesn't need to be limited to just their home planet. One species can have colonized (or conquered) multiple planets. If everybody on these planets still speaks the same language, it's also this trope.
The same can happen in a fantasy setting where all civilisations on the planet, even if they are from different races, still speak the same language. This is less likely to happen on a Multicultural Alien Planet, since different cultures often come with their own languages, though the two aren't mutually exclusive.
Differs from Common Tongue in that a setting with a Common Tongue acknowledges that other languages exist on the planet and are still being spoken within specific groups, even if no examples appear in the work. However, sometimes a Single Language Planet can be explained by the planet having used a Common Tongue for so long that all other local languages were forgotten.
Like many alien characterization tropes, this is at least partly to save the creator some time, especially if they go to the trouble of designing a full-on Conlang for the aliens.
Often related to Planet of Hats, Planetville, Planetary Nation, and It's a Small World, After All. Contrast with Aliens Speaking English. Compare Multicultural Alien Planet.
Examples:
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Anime & Manga
- Dragon Ball:
- Every race on Earth speak the same language.
- The Namekians all speak the language of the same name. Justified as Namek is a rather small planet, and there are only a hundred Namekians left after a series of catastrophes nearly wiped out their race save for Guru, the only survivor who then hatched all the current Namekians.
Comic Books
- Superman: As Kal-El and Supergirl learned in The Krypton Chronicles, everyone on Krypton spoke the same language, called Kryptonese, and used the same writing system during millennia.
- Wonder Woman (1942): The Saturnians speak the same language across the several moons their empire encompasses, though their agents stealing humans as slaves have also learned English.
Fan Works
- Dungeon Keeper Ami: It seems everyone on Adushul speaks the same language, which makes sense because they all know the same gods, so global communication is useful, even if they're split between good and evil.
- Star Trek: Enterprise fanfic Radiant Orb: Parodied when Archer thinks that humans are the only species that have more than one language.
Films — Live-Action
- Avatar has this enforced. On Pandora the native population, despite being split into numerous clans, only has one language, the lì'fya leNa'vi. There's no variance between individual clans. Partly justified, because since there is a planetary superintelligence on Pandora, and since every living being is more or less linked to Eywa, this could be a reason why there's a uniform language on this world.
- Played with in Avatar: The Way of Water, the island Metkayina speak the same verbal language as the forest-dwelling Omatikaya, but the Metkayina also have a signed language that they use underwater that the Omatikaya characters have to learn. And the Tulkun, a wholly different species, have their own language that sounds like whalesong.
Literature
- Earth Girl features a future humanity, spread across over a thousand planets but using only one language, called Language. We don't know how this language should appear as the characters appear to be speaking English but a newspaper from 23rd-century New York will apparently need to be translated before anyone can understand it. The prequel series, Scavenger Alliance, establishes that Language was created and enforced by the Wallam-Crane dynasty in an attempt to unify humanity. It is mentioned that some worlds in Alpha Sector (the first settled) still use some of the old languages, while Gamma Sector has its own dialect.
- John Carter of Mars: Played straight with the oral language on Barsoom, which is global and spoken by all intelligent races, even those that live in remote areas and rarely, if ever, have contact with the outside world. There don't even seem to be any accents or dialects. Averted with the written language, which can differ greatly between the various city-states and species living on the planet.
- "Lumbanico, the Cubic Planet": Everyone on the titular planet speak the same language, even though the different clans and enclaves have lived isolated from each other for seven hundred years.
- Star Wars Expanded Universe: Huttese is not only the language spoken by all Hutts, but due to their influence and the size of their territory, many other species also speak Huttese as a Lingua Franca beyond Basic.
- M.C.A. Hogarth writes Con Langs for most of her settings but rarely does more than one for a given species.
- Kherishdar: The Ai-Naidar live on five planets but the Kherishdar Empire has taken careful steps to ensure their one language doesn't differentiate, with a group of scribes dedicated to chronicling all new words and propagating them if deemed suitable.
- Paradox: Every species has a single language, but most of them are Transhuman Aliens that left Sol just a few centuries before the establishment of the Alliance and many of them have almost completely abandoned their native tongues for Universal. The Chatcaava, one of the few true aliens, do turn out to have a Classical Tongue eventually though.
- Shadows of the Apt: Played with. Characters have no problem speaking to each other no matter which part of the world they hail from or which part of the world they're visiting, even in places that have been isolated for centuries or millennia. However, this doesn't apply to written language, which has developed along many (often wildly different) lines, even though they all transcribe the same phonetics. This goes unremarked on until the short story For Love of Distant Shores, which explains that spoken language is an innate, biological characteristic of humans in this setting, which develops in children the same way for everyone, everywhere, and in fact has only gotten more regular over time. As a result, when the protagonist reaches the new continent and runs into some of the natives, she can't understand why they're just making weird sounds with their mouths until she realizes that they speak a different language, which is a completely alien concept to her.
- Star Trek:
- Averted in the Star Trek reference work "The Klingon Dictionary", which noted that there were several different dialects of Klingon. Some of the dialects only had minor differences compared to the "official" language but others were so different that they could almost be considered separate languages. The dictionary went on to say that most Klingons tried to be fluent in several dialects.
- Averted in the Star Trek: Deep Space Nine novel "Warchild" as a Bajoran Vedek reminds Sisko that there are several different Bajoran languages, some of which are only used within the temple.
Live-Action TV
- Star Trek:
- All Vulcans appear to be speaking the same language (when not translated through the Universal Translator).
- The Klingon language is a well-known example, to the point that it's become an official language in Real Life.
- Babylon 5: The Narn and Centauri seemed to be monolingual. Averted by the Minbari, though. That race had three languages—"Light", "Dark", and "Grey"—one each for the Worker, Warrior, and Religious Castes respectively.
- Firefly: English seems to be universally spoken, albeit with a number of Chinese words (mainly curses) included. Written Chinese characters are often seen on signs and such, however.
- Defiance: Each Votan species has one language (or at least Castithans, Indogenes, Irathients, and Volge do). Even though some species came from the same homeworld and it may be justified in that only a small part of their populations was left on the Arks. And of course they speak English most of the time since formerly American humans are the majority in Defiance.
Religion & Mythology
Video Games
- Halo: The member species of the Covenant all speak in a single language respective to their race. Overlaps with Starfish Language in that the majority of these languages are very alien, such as the Jiralhanae/Brute language which consists of barks, roars, and howls, or the Huragok/Engineer language which can best be described as a mixture of whale song humming and bird chirps.
Web Animation
- RWBY: All inhabitants on Remnant speak the same language. There even doesn't seem to be language specific to the Faunus. This can be justified because the constant attacks of the Grimm mean that people are constantly uprooted and on the move, making it difficult for local dialects to take hold. Also, due to modern technology, worldwide communication is possible.
Webcomics
- El Goonish Shive: All Uryuoms that are present on Earth seem to speak the same language, Uryuomoco. It helps that they can instantly teach it to others through telepathic transfer.
Western Animation
- Avatar: The Last Airbender: Regardless where the Gaang travels, from the South Pole to the North Pole, everybody speaks the same language and uses the same script. Not only that but other than some popular expressions, Aang's way of speaking isn't any different even though it should be 100 years out-of-date.
- The Dragon Prince: Even though they're separated by a river of lava and a generations-long enmity, the Humans from the Five kingdoms on one side and Elves and Dragons in Xadia on the other side all speak the same language. The different elf tribes do have different accents, but there's no indication that these accents come from an underlying different language.
- She-Ra and the Princesses of Power: Everybody in the Firght Zone, the Resistance, and assorted freeholds all speak the same languages. This also transcends time since Mara died a thousand years ago, but in her appearances in flashbacks or holograms she recorded while alive she speaks the same language as Adora and Glimmer. And Horde Prime can communicate fine with Glimmer and Catra even though Etheria has spent a thousand years in a different dimension to the one Horde Prime came from. The First Ones had a different written language, admittedly, but the spoken language seems to be the same.
