Classical Tongue

A language that isn't known/used by the common man. It may be dead/(mostly) forgotten, or is spoken by nobility/scholars/any elite or knowledgeable/educated person in general. It'll probably be both. A few words from it might be used to denote something special, or is used for something or someone's name, but don't expect the common masses to generally use it (anymore). It's often an Expy of Latin, if it isn't Latin itself.

The language may also be some kind of holy language spoken by those in a religious order, which makes the comparison to Latin all the more obvious. In these instances, see Sacred Language.

A Language of Truth or Language of Magic often doubles as one.

Compare Smart People Know Latin. Contrast Common Tongue.

See also: classical language on The Other Wiki.

Anime and Manga

 * Ancient Belkan (which is suspiciously similar to modern German) in the Lyrical Nanoha franchise is spoken only by a few old-fashioned Knights and their Devices.
 * In Scrapped Princess it turns out that the old language is English. For example, the name of their continent, "Dustvin", is a corruption of "Dustbin".

Film

 * The Divine Language spoken by esoteric priests and sexy living MacGuffins in The Fifth Element.

Literature

 * "High Speech" from The Dark Tower, which is the language of Gilead, the city of Gunslingers, and is considered a "civilized" language compared to the Low Speech of everyday communication. It's an "old" language in the sense that, by the time of the first novel (The Gunslinger), Roland is the last of the Gunslingers and thus one of the few people left in the world who actually can speak it.
 * High Valyrian from A Song of Ice and Fire.
 * During the Archprelate's funeral in the Elenium, there's a reference to the prayers and chants being in an archaic form of the Elene language that hardly anyone present understands.
 * The High D'Haran from The Sword of Truth series.
 * Several in The Lord of the Rings: Westron, the Lingua Franca and Common Tongue of the series, is derived from Adûnaic, the language of the Númenóreans. Quenya serves in the same capacity for the Elves.
 * Dwarves never speak their own tongue to outsiders except their Battle Cry(and presumably the hearers are expected to soon be dead). Dwarvish is a ceremonial language and is exactly the same in the time of Lord of the Rings as it was in Durin's time. When talking to outsiders and sometimes even among themselves they use Common Speech. And if an outsider knows Dwarvish that is a sure sign that he is considered to be a very honored friend and possibly that he has Gone Native.
 * The Old Tongue in The Wheel of Time, precursor to the Common Tongue used in the books. It was similarly the universal language in its own time, and is now used mainly by nobles to show off their status, much as Latin was in the real medieval age.
 * There's The Inheritance Cycle's Ancient Language. As an added bonus, it's a Language of Magic.
 * In Time Enough for Love, which takes place in the 43rd century, more or less, English is a dead language, which Lazarus Long insists on speaking when dealing with the Howards.
 * At the end of Gaudy Night, Wimsey makes his last marriage proposal to Harriet Vane in Latin and she accepts in the same language ("Placet," meaning "it pleases"). They're both highly educated graduates of Oxford University, where the novel is set. The event is special, in that Wimsey unsuccessfully proposed marriage to Harriet (in the vernacular) repeatedly over the course of several years and several novels.

Live Action TV

 * Downplayed on Grimm when Nick needs a text translated that is written in a medieval dialect of German. Monroe explains that the dialect has not been used in a long time but he can still translate it because it is close enough to modern German.
 * It's mentioned in one episode of Star Trek: The Next Generation that French has become a dead language.
 * Old High Gallifreyan in Doctor Who.

Tabletop Games

 * Dungeons and Dragons
 * Forgotten Realms has Auld Wyrmish -- the language of ancient dragon civilization on which different species talk with each other, while each kind itself uses derived dialects. Much the same applies to Giant, which is not the same as dialects e.g. stone giants or verbeeg use among themselves. Thorass or "Auld Common" is ancient written trade language from which "Common" is derived, still used for official records in some lands.
 * High Drow is an ancient dialect known mostly to the drow clergy, different enough that they sometimes use it for secret communication over the heads of lay worshipers.
 * High Gothic of Warhammer 40000. It's rendered as Dog Latin in the books, but Word of God is that it's just a Translation Convention.
 * Among Darrians in Traveller the Old Script Yaser Te-yulep is used for poetry and legal documents and the Roman Script Rome Te-yelup is used generally.

Video Games

 * Ancient Hylian in The Legend of Zelda: The Wind Waker.
 * Several in The Elder Scrolls. The Dwemer language is mostly lost, rediscovering the dragon language is a plot point in Skyrim, as is the Falmer language.

Web Comics

 * In Ears for Elves, some children argue about the proper pronunciation of "Taurë", the word for "wood elves" in the Forgotten Tongue. It's the first part of the name of the Taurëcuiva Festival.

Western Animation

 * Futurama has a Running Gag where French has become one by the year 3000.

Real Life

 * Latin, of course.
 * Ancient Greek is also a real life example (in fact as a rule of thumb, Latin is favoured by Lawyers and Greek is favoured by scientists).
 * Oddly, some jokingly refer to the French language as a dead language, when it's obviously not.
 * It was used as a language among nobility in some places, like Tzarist Russia.
 * Sanskrit also fits the bill, and it's even used as a ceremonial language in Hindu religious rituals and Buddhist practice in the forms of hymns and mantras.
 * The Other Wiki has a long list of extinct languages.
 * It also has several Sacred Languages used by various religions. Including: Latin, Sanskrit, Koine Greek, Aramaic...
 * In the British Isles there are those who desperately try to keep alive Welsh, Cornish, Scottish and Irish Gaelic. Arguably, all are examples.
 * Hebrew. Interestingly it has revived as an everyday speech as a deliberate social engineering project of the Zionists. Many Orthodox(who have always had to say the least, a rather ambiguous relationship with Zionism anyway) disliked this considering Hebrew a holy language and Yiddish the common language of the Jews.