While a universal translator seems unlikely, due to the apparent need for telepathy, scientists continue to work towards similar real-world technologies involving small numbers of known languages. See machine translation and speech recognition for discussions of real-world natural language processing technologies.
The existence of a universal translator is sometimes problematic in film and television productions from a logical perspective (for example, aliens who still speak English when no universal translator is in evidence and all characters appear to hear the appropriately translated speech instead of the original speech, the ability to speak in the language when direct translation is possible), and requires some suspension of disbelief when characters' mouths move in sync with the translated words and not the original language; nonetheless, it removes the need for cumbersome and potentially extensive subtitles, and it eliminates the rather unlikely supposition that every other race in the galaxy has gone to the trouble of learning English.
In the universe of "The Hitchhiker's Guide to the Galaxy", universal translation is made possible by a small fish. The fish is inserted into the auditory canal where it feeds off the mental frequencies of those speaking to its host. In turn it excretes a translation into the brain of its host.
The book remarks that, by allowing everyone to understand each other, the babel fish has caused more wars than anything else in the universe.
The book also explains that the babel fish could not possibly have developed naturally, and therefore proves the existence of God as its creator. Since God needs faith to exist, and this proof dispels the need for faith, this therefore causes God to vanish "in a puff of logic".
Also, a new race called the Orz was introduced in Star Control II. They presumably come from another dimension, and at first contact, the ship's computer says that there are many vocal anomalies in their language resulting from their referring to concepts or phenomena for which there are no equivalents in human language. The result is dialogue that is a patchwork of ordinary words and phrases marked with *asterisk pairs* indicating that they are very loose translations of unique Orz concepts into human language, a full translation of which would probably require paragraph-long definitions. (For instance, the Orz refer to the human dimension as *heavy space* and their own as *Pretty Space*, to various categories of races as *happy campers* or *silly cows*, and so on.)
In the other direction, the Supox are a race portrayed as attempting to mimic as many aspects of other races' language and culture as possible when speaking to them, to the point of referring to their own planet as “Earth,” also leading to confusion.
In Star Control III, the K’tang are portrayed as an intellectually inferior species using advanced technology they do not fully understand in order to intimidate people, perhaps explaining why their translators’ output is littered with misspellings and nonstandard usages of words, like threatening to “crushify” the player. Along the same lines, the Daktaklakpak dialogue is highly stilted and contains many numbers and mathematical expressions, implying that, as a mechanical race, their thought processes are inherently too different from humans’ to be directly translated into human language.
A notable exception to this rule are the Goa’uld, who occasionally speak their own language amongst themselves or when giving orders to their Jaffa. This is never subtitled, but occasionally a translation is given by a third character (usually Teal’c or Daniel Jackson), ostensibly for the benefit of the human characters nearby who do not speak Goa’uld. The Asgard are also shown having their own “language” (apparently related to the Norse languages), although it is in fact English played backwards. (see Hermiod).
Given that the Asgard and the Ancients were existent in the same 'Alliance of Four Great Races', it is possible that English was used as the universal language mentioned in "The Torment of Tantalus". Considering also that both races play or played a pivotal role in the interplanetary relations of several galaxies, as well as dealings with the Goa'uld, this could be the explanation of the existence of English as a semi-universal language. This would mean that, instead of other planets speaking Earth-developed English, Earth is in actuality speaking Alien-developed English, and as the Asgard and Goa'uld are cited as the source of a number of other Earth languages, would seem to be the case.
Improbably, the universal translator has been successfully used to interpret non-biological lifeform communication (in the Original Series episode “Metamorphosis”). In the Star Trek: The Next Generation (TNG) episode “The Ensigns of Command,” the translator proved ineffective with the language of the Sheliaks, so the Federation had to depend on the aliens’ interpretation of Earth languages. It is speculated that the Sheliak communicate amongst themselves in extremely complex legalese. The TNG episode “Darmok” also illustrates another instance where the universal translator proves ineffective and unintelligible, because the Tamarian language is too deeply rooted in local metaphor.
Unlike virtually every other form of Federation technology, Universal Translators almost never break down. Although they were clearly in widespread use during Captain Kirk’s time (inasmuch as the crew regularly communicated with species who could not conceivably have knowledge of Standard English), it is unclear where they were carried on personnel of that era; possibly they existed as implanted devices before that practice was deemed too potentially dangerous and discarded by the era of Next Generation.
The episode “Metamorphosis” was the only time in which the device was actually seen. During Kirk's era, they were also apparently less perfect in their translations into Klingon. In the sixth Star Trek film, the characters are seen relying on print books in order to communicate with a Klingon military ship, since Chekov said that the Klingons would recognize the use of a Translator. Actress Nichelle Nichols reportedly protested this scene, as she felt that Uhura, as communications officer, would be fluent in Klingon. The novelization of that movie provided a different reason for the use of books: sabotage by somebody working on the Starfleet side of the conspiracy uncovered by the crew in the story, but the novelization is not part of the Star Trek canon.
By the 24th century, Universal Translators are built into the communicator pins worn by Starfleet personnel, although, since crew members (such as Riker in the Next Generation episode “First Contact”) have spoken to newly encountered aliens even when deprived of their communicators, some other factor must also be at work.
In some episodes of Star Trek: Deep Space Nine, we see a Cardassian universal translator at work. It takes some time to process an alien language, whose speakers are initially not understandable but as they continue speaking, the computer gradually learns their language and renders it into Standard English (also known as Federation Standard).
Ferengi customarily wear their Universal Translators as an implant in their ears. In The Star Trek: Deep Space Nine (DS9) episode “Little Green Men,” the humans without translators are able to understand the Ferengi once the Ferengi get their own translators working. Similarly, throughout all Trek series, a Universal Translator possessed by only one party is able to audibly broadcast the results within a limited range, enabling communication between two or more parties, all speaking different languages. The devices appear to be standard equipment on starships and space stations, where a communicator pin would therefore presumably not be strictly necessary.
Since the Universal Translator presumably does not physically affect the process by which the user's vocal cords (or alien equivalent) forms audible speech (i.e. the user is nonetheless speaking in his/her/its own language regardless of the listener's language), the listener apparently hears only the speaker's translated words and not the alien language that the speaker is actually, physically articulating; the unfamiliar oratory is therefore not only translated but somehow replaced. This implies the Universal Translator may work at least partially on a telepathic level. There is no explanation why we could hear not-translated words like klingon.
Another exception is the “translator microbes” from the Farscape series, which were probably inspired by the Babel fish.
In K. A. Applegate’s famous science fiction series, Animorphs, all Andalite warriors have miniature translator chips in their brains, which enable them to readily understand any spoken alien language. This is mentioned in The Hork-Bajir Chronicles and The Andalite Chronicles. However, in the series most aliens possess “thought-speak,” a type of telepathic communication, which operates more on the essence of thoughts than the words themselves; thus, an alien can thought-speak in their own language, but everyone hears it as their own. When morphed into a non-thought-speaking creature, such as a human, aliens seem to gain the ability to speak English, possibly due to a translator. Some aliens also seem to speak English without a translator, such as the free Hork-Bajir (who could have picked it up when they were Controllers interacting with Human-Controllers) and the last Arn (who explains that he studied the language, learning it quickly while in orbit). There is also a lingua franca called galard, used in communication between aliens capable of speaking it.
In the video game The Legend of Zelda: The Minish Cap, Link learns to talk to the inch-high Minish race by eating a Jabber Nut acquired in the Minish village. Minish seems to be the only language that Jabber Nuts enable the user to speak, as otherwise the Minish would eat the nuts themselves and learn to speak English. When Link is in his Minish form, he can talk to animals such as dogs, cats, chickens, cows and horses; it is unknown whether this is an effect of the Nut or of Minish DNA.
In the game Tombi, the titular character learns how to speak to Dwarves by biting one on the head several times. If one tries to speak to the Dwarves before the language is learned entirely, the Dwarves appear to speak a mixture of the language being played in and gibberish.
In anime and manga series Doraemon, one of recurring items is translation konjac, who enable everyone who eats it to be fluent in any language on current universe.
In the videogame Sid Meier's Alpha Centauri, the universal translator is a secret project which gives the player two new technologies, which are supposedly taken from the abandoned monoliths the alien race left behind.