Common Tongue

In Real Life, there are more languages than there are cultures speaking them. There are dead languages, sign languages, dialects, slang, and a thousand other variations. People ten miles away from each other might not understand a word each other says.

This makes communication difficult. So, many speculative fiction writers use a shortcut: Everyone speaks the same language.

Usually called Common or Basic, this is a baseline language that is used by the vast majority of the setting. Oftentimes, it is the human language, since humans are almost always the most wide-spread race, and other races will have their own "Common" language that all their members speak. All dwarves will speak Dwarven, all elves Elven (or Elvish) and so on.

If it is never stated explicitly that everyone is speaking the same language, it might be a case of Translation Convention or even Translator Microbes instead. See also Aliens Speaking English and Animal Talk.

Fan Fic

 * The Basalt City Chronicles averts this; in the Empire of Smilodons, it's said that there's a language for every island and a dialect for every village. Some cultures even have more than one language, for example the Deltharians (most of whom are deaf) have a spoken language used by the few who can hear.

Film

 * Star Wars uses Basic, the language of the Galactic Republic. Nearly everyone understands it, even aliens that lack the ability to speak it. Likewise, most aliens have one language that they speak constantly. Interestingly, multilinguism is quite common--Han, for example, speaks Huttese, Wookie (though he sounds really stupid when he tries), and Rodian.

Literature
""And Morporkian is something of a lingua franca even in the Klatchian empire. When someone from Hersheba needs to trade with someone from Istanzia, they will undoubtedly haggle in Morporkian.""
 * The Trope Namer is The Lord of the Rings, where Gandalf translates an Elvish inscription into "the Common tongue." Elves speak Elvish, dwarves Dwarvish, and halflings...Common, for some reason. Tolkien, it should be noted, was a language professor, so his constructed languages made a good deal more sense than normal.
 * Tolkien's work may be the trope namer, but Tolkien's handling of languages was subtle enough that later fantasy works seem like Flanderizations in comparison. The Common Tongue in the time of the novels is Westron, which evolved from Adûnaic, the language of one of the tribes of men who joined the Eldar in Beleriand in their fight against Morgoth during the First Age; Adûnaic later became the language of Númenor, who later became the major imperial power in Middle-Earth, and then the language of the kingdoms of Arnor and Gondor, where it changed quickly and evolved into Westron under influence from Elvish languages. Most races and human nations in the story have their own languages (e.g., the Rohirrim, who speak a more archaic version of Westron, represented by Old English in the actual book), but a substantial number of these people can speak Westron as a lingua franca, in the case of dwarves, they actually refuse to speak their language to nearly anybody who's not a dwarf (in all of history, they only taught it to some of Celebrimbor's people...and Eöl, although they don't like talking about that). Hobbits are recorded to have spoken three different languages through their history: the northern hobbit races originally spoke a language closely related to that of Rohan, while the southern ones spoke a language related to that of Dunland. When they settled in Arnor, they all came to speak Westron.
 * Also, during the First Age as recounted by The Silmarillion, Sindarin ("Gray Elven") was the Common Tongue.
 * Clan of the Cave Bear: the various individual camps of Clan people have their own languages but there is a formal Clan language that everyone can "speak" (it's non-verbal); when Ayla meets Jondalar she wants to learn the human Universal language and can't understand for a while that there isn't one.
 * In the Ender's Game universe, there is a common language based on English called Stark, short for Starways Common.
 * At the same time, many cultures, who have spread out among the stars, have retained their own languages, even though they still use Stark when working with computers or sending messages. When traveling to Lusitania, settled by descendants of Brazilians, Ender tries to learn Portuguese, and the book is peppered with Portuguese words and phrases. Several Swedish words are also used constantly, specifically those dealing with the so-called "Hierarchy of Foreigness".
 * Animorphs has Galard, a Yeerk language briefly mentioned a couple of times. The Yeerks designed it to be spoken by all kinds of different host bodies with unusual vocal chords.
 * In the Liaden Universe, the common language is called Trade, and that's what it's mostly used for.
 * In Hellspark by Janet Kagan, the common language is called GalLing' (presumably from "galactic lingua franca"); it's an artificially-constructed language, and one of its design features is that it only uses phonemes common to all human languages, so that anybody can speak it without difficulty.
 * This is a puzzling matter in The Wheel of Time series, and one of the greatest flaws in what is otherwise a masterwork of worldbuilding. Everyone in the entire world speaks the same language, with minor dialectical variations, including people who have been completely isolated for a thousand years. No explanation for this is ever given.
 * It is somewhat explained in that all the languages are based on a common ancestor, known, quite originally as the Old Tongue. Also, there are many, many accents, and some places, notably Illian, even use different syntax. As for the Seanchan, remember they were invaded and taken over by people from the main continent, who may have forced their language closer to that of other places. Still, having Common is probably just for the sake of convenience, given how there are characters from just about every country in the cast.
 * Also, according to Word of God, literacy within the setting is unusually high due to the Printing Press being one of the few technologies to survive the Breaking of the World. Cuts down on linguistic drift a bit.
 * Discworld (specifically, said by Lord Vetinari, in Jingo):


 * In the Foundation series everyone speaks Galactic Standard, albeit with some dialects.
 * The characters in Tamora Pierce's Tortall Universe novels, most characters speak a language called Common. In the Circle of Magic universe, characters in different countries speak different languages, but everyone also seems to know how to speak Imperial.
 * In the Robotech Expanded Universe, it's revealed that though there are many languages still spoken amongst the Sentinal's races, Zentraedi has become a sort of common tongue that everyone can understand. This is justified because the Robotech Masters used the Zentraedi as soldiers to create their empire, and thus the language was spread.
 * The Humanx Commonwealth universe has "symbospeech", an In Universe Con Lang that became a common tongue through serendipity. Shortly after humans and thranx met one another, they spent some time working out a language that was easily pronounceable by both species, as they had wildly different vocal apparatus, and the thranx language incorporated significant body language aspects in addition to vocalizations. When additional species were encountered, symbospeech was found to be functionally pronounceable by them, too, and thus became the de facto galactic language.
 * Anne Mason's Kira Warden novels have "the interplanetary language". Theoretically, most people know how to speak it; in practice, a lot of them are pretty bad at it, and it's not very good at nuance, providing lots of work for interpreters like the protagonist.
 * In A Song of Ice and Fire, the main language of Westeros ("translated" as English) is actually referred to as the Common Tongue. It is contrasted with the Old Tongue (i.e. Old English), spoken by the original people who lived on the continent,as well as High Valyrian (i.e. Latin/Romance languages), spoken by foreign conquerors.
 * In the Uplift series the ridiculously organized and stagnant culture of galactic civilization has resulted in at least twelve different Galactic languages (numbered 1-12) designed to accommodate the wide variety in vocal structures, humans seem to have the easiest time with Gal 7. One of the assorted ways that Earthclan is different from the other oxy-breathing races is that they have languages other than Galactic, mostly Anglic and Trinary, a poetic language designed for dolphins.
 * Generally, Harry Harrison's novels set in the future will have Esperanto (a failed Real Life attempt at making one language out of many) as the language of the old Empire and as lingua franca of all worlds. In the Deathworld novel The Ethical Engineer, Jason finds himself on a Lost Colony and tries to converse with the locals. After some attempts, he quickly finds out that their language is a degraded form of Esperanto and is easily able to communicate.

Live Action TV

 * Commonly Zig-Zagged in Buffy the Vampire Slayer and Angel. Almost every demon speaks English, even ones in alternate dimensions, until it's more dramatic for one not to.
 * Babylon Five had Interlac, which I think was math-based. (As it went, most races seemed to learn English easily enough, though admittedly most examples were diplomats and traders.)
 * There was also a situation in Stargate SG 1 where four ancient races used holographic displays of various periodic elements as some kind of universal language, though the intended effect was a little vague.
 * Of course, the way they chose to represent the elements (as orbiting spheres) doesn't sound like something various alien races would come up with on their own. Especially since the Asgard claim that they can't think primitively.
 * Given the language itself is thousands if not millions of years old, it was most likely a time when the Asgard could "think primitively."
 * Star Trek: The Federation had Federation Standard and the Klingon and Romulan empires had the racial languages of the Klingons and Romulans.
 * Although, the line between this trope and Translator Microbes was frequently blurred.
 * In Doctor Who, this trope is subverted- the TARDIS is translating for both the Doctor and his companions. This comes across to the audience as British-accented English.
 * Except for the Ninth Doctor, who had a Northern accent.

Tabletop Games

 * Dungeons and Dragons, being heavily based on The Lord of the Rings, uses this extensively. But tries to not give in completely as it has language-related magic. Specific settings are likely to have a "Lingua Franca" and a handful of specific languages.
 * Forgotten Realms subverts this by having several "trade languages" even on Faerûn. Usually people can talk to each other, but on the larger scale there are Common "common" (Heartlands' dialect of Planecommon), Kara-Tur "common", Undercommon (mix of Dwarven, Gnomish, Low Drow, Upper Common etc), Auld Wyrmish ("common" across dragon subspecies). Other continents may have their own "common" languages, like Midani of Zakhara. While many specific cultures retain their own tongues still, though some reduced to dialects of "common". So learning all half a hundred or so present tongues (like Wemic or Gnomish speech) is unnecessary, but doesn't 'Comprehend Languages' spell look worth learning now?
 * Simplified in 4th edition, (no surprise there) for the most part there are only ten languages, with common being the trade language. There are however 7 other languages for different regions.
 * Traveller. Galanglic was the official language of the Third Imperium.
 * In the Spacemaster setting Privateers, the language Species Standard is spoken by all of the known intelligent races.
 * The "Gothic" language in Warhammer40000 serves this purpose for the Imperium, acting as a way for cultures from different worlds to communicate. There is also High Gothic, which is used for official purposes and has a role similar to Latin in medieval Europe in that no-one actually uses it as a first language but scholars and those of high rank are expected to know it.
 * Dark Dungeon RPG, supplement Samaris, island of Adventure. In the world of Yaddrin, the Common Tongue is spoken by most merchants and travelers.

Video Games

 * Mass Effect makes heavy use of Translator Microbes in the form of computers that need to be regularly updated for new languages, as practically every species in the setting is as linguistically diverse as humans. There is, however, a "trade tongue", which Shepard refers to as "Galactic" at one point -- a simplified artificial interspecies language, essentially Space Esperanto.
 * The Longest Journey gave us Na'ven or Alltongue, a magical language spoken in all of Arcadia (a parallel universe). Its omnipresence is justified with the fact that you can become a fluid speaker after listening to it for just a few minutes, as April does upon her first visit to Arcadia. It's magic.
 * Interestingly, Zoë from the sequel doesn't appear to need to listen for several minutes before learning the language. Perhaps it's because.
 * Common in War Craft games. It is primarily the language of humans, but nearly everybody can speak either it or Low Common, which sounds like a Hulk Speak version of Common. Now, it makes sense that races allied with humans would learn their language, and the orcs could've picked up how to speak it during the war or while in internment camps, but it makes less sense when tauren in Warcraft 3 can communicate with humans and orcs despite never meeting either one. In World of Warcraft Common is the Alliance universe language (the Horde has Orcish) and is not understood by Horde races. However this is because of game mechanics (blood elves and undead should definitely be able to speak it, as well as many orcs and goblins) and there are still NPCs in game that can be understood by all factions.
 * It's implied that the Forsaken lose the ability to speak the languages they knew in life (a tailor in the undead city in WoW says his former family were speaking a language "I no longer understand".
 * However, a later RPG book states that Forsaken can still speak Common, but refuse to do so to distance themselves from their old lives. Instead they speak Gutterspeak, formerly the thieves' cant of Lordaeron.
 * In Dark Star One, all alien races use a language called Terra (read: English) in order to make communication between them easy.

Webcomics

 * Schlock Mercenary actually has five common languages: Galstandard West, East, Eight, Brown, and Peroxide. Given recent revelations that Galstandard Peroxide is only spoken by ocean-dwelling creatures, it seems as though each language is tailored to a specific voice and vocal type.
 * Most units in Erfworld speak Language, but Natural Allies have their own (unnamed) languages, and only a few members of each tribe speak Language.

Web Original

 * English is the common language of the entire universe in Chaos Fighters, as explained here.
 * The First Federation of Orions Arm attempted to standardize "Anglic", but once the Feds lost power Anglic evolved into a family of languages several times more diverse than the current Indo-European family.

Western Animation

 * All the various cultures in Avatar: The Last Airbender appear to share the same language. From pole to pole and around the world.
 * He Man and The Masters of The Universe and She Ra Princess of Power: Pretty much all of the countless species and races on Eternia speak or at least understand English. Furthermore the same applies to Etheria, Trolla, Primus... pretty much any planet or dimension the characters encounter, including Earth making this also a case of Aliens Speaking English.

Real Life

 * English is the most universal example of this trope In Real Life, due mostly to the very expansive English speaking British Empire and later the global dominance of the United States in the latter half of the 20th century. Although it is not the most natively spoken language, it is the most often taught as a second language, and thus the most widely spoken. This is confirmed by international treaty, which stipulates English as the official language of aerial and maritime communications, and is considered a working requirement for various scientific fields. They don't call it "The world language" for nothing.
 * Esperanto is an attempt at this.
 * Transpiranto is a parody of this.
 * Further constructed languages attempting this: Interlingua and Ido.
 * Also Loglan and its derivation Lojban (short for "logical language" in English and Lojban respectively). The former of which was mentioned in a couple of Robert Heinlein's novels for use with A Is.
 * As part of the legacy of the conquests of Alexander the Great, Greek used to be pervasive throughout the old Eastern Roman Empire, to the point where even The Bible was translated into it so that it could be understood by Hellenized Israel. Hellenistic Greek is actually the Trope Co-Namer, as its most basic and used variety was known as Koine Dialektos (literally, the Common Tongue).
 * Russian, conversely, enjoyed this status in the Communist bloc; learning Russian there was like learning English in Europe. It still works that way in the countries of the former Soviet Union.
 * In newer generations, not exactly anymore.
 * Russian also functions as a common tongue language in outer space.
 * There have been attempts to "reconstruct" the original Sioux language, before the splitting into five dialects. This results in things like a sound not unlike the Japanese r instead of the /l/, /d/, and /n/ that are so famous, though no r-like sound exists in modern-day Sioux languages.
 * In past centuries, French was the language of choice for international communication. Many French are still bitter about this.
 * The language people know as "Chinese" is actually only Mandarin, which is spoken largely everywhere due to it being taught as part of the official curriculum. Otherwise, people in China speak a large family of languages sufficiently dissimilar that knowing one doesn't help in understanding another.
 * However, their common descent (from the Old Chinese language spoken up to about the Warring States Period) means that learning them is easier once you know one of them; ask native English speaker who has taken French and then Spanish (or any other combination of Romance languages) how much easier the second language was than the first for a comparable phenomenon.
 * Chinese linguistic unity is further increased by its logographic (each symbol represents a word) system of writing; the same glyph would be pronounced differently in each language, but usually remains the same. Therefore, a written language independent of speech, known as Classical Chinese, developed, serving as a Common Tongue (or Common Pen?) for the educated not only in China, but also countries under Chinese influence (Japan, Korea, and Vietnam). However, Classical Chinese was based on Late Old Chinese and thus did not reflect several features of more modern Chinese languages, including pronunciation and grammar. Classical Chinese fell out of use shortly after the Xinhai Revolution of 1911, but the Republic of China (Taiwan) used it well into the 1970s for certain government documents.
 * India is in the same boat as China: there are thousands of languages, but almost everybody there speaks Hindi or English.
 * After India became independent, there was a movement to purge British influences including English. The return to traditional languages failed because it was far too useful to have a single standard language that most educated people already knew. Economic reforms in The Nineties, which opened India to the wider world economy in which English is a huge advantage, put the final kibosh on any attempts to remove English from the country (and gave rise to the Operator From India trope).
 * In even earlier centuries, Latin was the preferred language for scholarly discourse. Latin is still in use by the Roman Catholic Church as its preferred language for edicts and internal documents.
 * The Italian language was developed by Dante Alighieri (author of The Divine Comedy) from a blend of Latin and an assortment of dialects spoken by the different city-states that currently make up the nation of Italy.
 * In general, when a large empire spreads its language around and then dies (either by being conquered or by splitting up into squabbling fiefdoms...or as often happens, both), the language usually starts to diverge into dialects, which dialects eventually become mutually unintelligible. However, that language may persist as a Common Tongue for the educated. One of the weirdest cases of this has to be the situation of Arabic. Nationalism and the printing press--factors that tend to stabilize languages--arrived at a time when the dialects of Arabic formed a continuum with only one significant break (between Western "Maghribi" and Eastern "Mashriqi" dialects, right about where the border between Egypt and Libya is today), and even that wasn't a complete one. Additionally, everyone in the region used various forms of Classical Arabic (the language of The Quran) for educated writing. As a result, Arab scholars developed Modern Standard Arabic, a streamlined form of Classical Arabic that also tends to get flavored with the dialect of the user, but which is universally understood by anyone who has been to school in an Arab country. However, people still speak their native dialects in all but the most formal circumstances; even in semi-formal situations, people will speak in their native dialect but use a lot of Modern Standard vocabulary. This last bit is the cause of a major fight among the Arab literati--many feel that this "educated colloquial" should form the basis of a new standard, abandoning the Classical entirely. Those who accept this view themselves bicker about whether one "educated colloquial" should be adopted as a single standard for all Arab countries (creating a new Common Tongue) or whether each country or group of countries should adopt their own standards (abandoning the idea of a single Arabic language altogether).
 * The Japanese dialects aren't so different that people would have too much trouble communicating with each other (aside from a few cases of Separated By a Common Language and when Okinawan get involved), but they still have hyojungo, or "standard language", that is roughly based on the Kantou dialect.