Language of Truth

A Language of Truth is a language or form of communication in which lies are inherently impossible or inexpressible. This is a frequent feature of Telepathy, but it can also appear for more overt languages, and is commonly associated with Languages Of Magic.

Often the Big Bad, or villains in general, will be exempt from this requirement.

Effort to make a Real Life Language of Truth can not work - not if you want it to be able to express every true statement. Consider:

"This statement can not be translated into a Language of Truth."

Is this statement true, or is it false?

Compare Can Not Tell a Lie, in which the inability to lie is inherent in individual characters.

Comic Books

 * Telepathy functions this way in Elf Quest. Unfortunately, the Big Bad's mind is so malicious that it's painful to receive her thoughts. So most elves use vocal speech with her instead, and she can lie just fine with words.

Literature
"While the Old Speech binds a man to truth, this is not so with dragons. It is their own language, and they can lie in it, twisting the true words to false ends, catching the unwary hearer in a maze of mirrorwords each of which reflects the truth and none of which leads anywhere."
 * The Old Speech in the Earthsea books, except to some extent for dragons:


 * The Inheritance Cycle: despite stating that elves are fond of half-truths and misdirection in the ancient language, it's never much of an issue. Although it is impossible to lie directly, lies can be told by thinking to yourself that you mean something else to what you are saying. Which partially explains how, in Eldest, Eragon is able to compose a completely fictional story in verse that tells of a battlefield romance -- because he was thinking of Arya and it felt true. It's similar to what happens when Eragon tries to demonstrate a spell to take a sort of magic photograph by using it on Arya. It was supposed to be a perfect photo, and it's supposed to be really hard to fool the spell, but it... came out different, because of the way Eragon felt about Arya.
 * Telepathy works this way in Ursula K. Le Guin's Hainish novels, except for the Shing (the aliens in City of Illusions).
 * Earths Children: The language of the Clan is like this. It's a sign language which supposedly involved so much careful observation of body language and expressions that any signs of lying would be obvious. Clan members can in fact lie, but can't get away with it, so nobody ever bothers -- to the point where the main character Ayla is confused by lying the first time she encounters it.
 * West of Eden: The Yilanè in Harry Harrison's trilogy have this. Learning to speak is what makes a Yilanè yilanè (capable of speech), and the thought is what causes the body motions that make up their language unless they are completely still. Humans who learn the language, however, are capable of lying in it.
 * Older Than Steam: In Gullivers Travels, the language of the Houyhnhnm is like this.
 * Nineteen Eighty-Four: Newspeak was a decidedly dark take on this, in that the goal was to make unsayable anything not deemed "truth" by The Party/Big Brother.
 * Obviously inspired by Orwell, The Book of the New Sun features the Ascians, a people whose language is composed of a bunch of Mao-esque revolutionary slogans. However, the book explicitly shows that Ascians are capable of uttering subversive statements even when bound by this language.
 * Hellspark by Janet Kagan: It is technically possible to lie in Jenji, but the language is structured to provide as much accuracy and detail as possible, and is backed up by cultural and religious penalties for lying. Several times characters refer to speaking in Jenji as synonymous to telling the whole truth and nothing but the truth.
 * In Sector General, the Kelgian speech and telepathic communication technically qualify. Kelgian languages are perfectly ordinary: the fun part is that their vocal speech is inextricably linked with the involuntary expression of emotions in the movements of their fur. The species developed no concept of lies. Or tact. They're not so hot on bedside manner, either. Telepathic contact is full awareness of the partner's psyche, making lies exceptionally pointless.
 * In Neal Stephenson's Quicksilver, philosophers are attempting to make a truly logical language which, when used correctly, makes it impossible to say things which are false. The idea is that it would work like algebra: valid algebraic transformations on correct equations always lead to other correct equations. This is a little bit different from some of the other examples, since it protects not only against the speaker lying, but also against the speaker mistakenly saying something wrong.
 * In EE Doc Smith's Lensman series, it is absolutely impossible to make an untrue state telepathically, and the recipient knows this, with absolute certainty. It is possible, however, to make a correct but uninformative statement, such as "I cannot divulge that."
 * Heralds of Valdemar: In Mercedes Lackey's Velgarth novels, only the dyheli, an artificial race of deer-like sapients with truly phenomenal telepathic abilities, can lie with Mindspeech.
 * In the Young Wizards series, the Speech is the language that defines the fabric of the universe. Even the Lone Power can't lie in it.

Tabletop Games

 * In Nomine has the divine language, which is impossible to lie in. The demons don't like that, so after the initial fall they create a butchered version, called Dark Celestial.

Visual Novels

 * Umineko no Naku Koro Ni has the red truth. If anything is said in red text, it must be true. If it's not true, it cannot be said in red text. As well as being vitally important to solving the mystery of the island, it gives us such gems as You are incompetent! and *cackle*cackle*cackle* hihihihihihihihihihihyahyahyahhahhahahaha-hhahahahhahhahhahhahhahhahha! In addition, if someone attempts to lie using the red text,

Webcomics

 * Gene Catlow: The "Sight of the Soul" telepathic communication works like this - since it's direct communication between souls, it doesn't allow for lies, falsehood, or even withholding the truth. However, recent comics have revealed that, at least at one point, there existed a type of etherical being with a mind so insane and fractured that it was capable of lying through the Sight of the Soul...
 * Tales of the Questor recommends that if you're going to cut a deal with a fae lord, Latin is the language to do it in. Being a dead language, meanings are fixed and specific. There's also a brief mention of floriography, or the Language of the Flowers, which allegedly carries similar weight.
 * Averted in Errant Story, where a telepathic fib by Sarine is crucial to seven more years of plot development.

Western Animation

 * The Fairly Odd Parents : Inverted; Timmy wishes to "always be right". Rather than making him only say correct statements, any claim he makes instantly becomes true.