Language comes in many different shapes and sizes, and their range of usability can change over the course of millennia. We have modern languages, used in daily life, and dead languages, some used as Classical Tongues, others as Sacred Language and other ones are known by archaeologists studying ruins or by paleolinguists.
And then there are these.
Lost languages are created when no-one is able to use and understand them, whether by the loss of speakers, lack of adequate translation, or simply by not having new people willing to learn and speak the language. Truth in Television very much applies here, given the vast multitude of languages that have emerged over the course of human history; it's exceedingly common for a language to only emerge among a small ethnic group and then die out as the people either die or adopt new languages (oftentimes by force). Most of these languages, once abandoned, become permanently lost thanks to no-one actually seeking out or archiving them, and ones that are archived may not necessarily be translatable. The Indus River Valley Civilization is one notable example of the latter: we have surviving writings, but nothing that allows us to actually decipher them. Thus, despite having easy access to the alphabet and writing methods of the extinct civilization, the language is lost by virtue of being un-translatable.
In fiction, lost languages are commonly used as a seemingly impenetrable puzzle for characters to face, with writings in this language typically containing vital clues to attaining a certain end goal. Thus, a good portion of the plot is dedicated to finding a means of deciphering these lost languages, be it through the discovery of a surviving speaker or a hidden Rosetta Stone.
- One Piece: The old language inscribed on the "Poneglyphs" has been lost to everyone except Nico Robin, one of the protagonists. It's because the Poneglyphs contain information about the Void Century, something that the World Government doesn't want people at large to know. The Poneglyphs are indestructible, so the WG went after people who can read them (mostly the scholars in Ohara) and then decimate them and their documents; Robin used to be an Ohara native and she's wanted by the WG because of it.
- In Atlantis: The Lost Empire, the Atlanteans are still able to speak their language, but have lost the knowledge to read it.
- Stargate: The Abydonians speak an Ancient Egyptian dialect, and Daniel Jackson, being an Egyptologist, is able to communicate with them once he figures out the pronunciation.
- By the end of The Rise of Skywalker, the Black Speech language of the Sith is implied to have become this, with the last remaining Sith and Sith-cultists dying on Exagol and with language-proficient droids such as C-3PO being expressly forbidden to translate it.
- Area 51: The fictional high runes and real rongorongo language of Easter Island feature in the island. In both cases, characters are able to partly or wholly decipher them (how is not explained).
- "The Double Shadow" by Clark Ashton Smith: A wizard finds a tablet inscribed with an utterly alien script, summons up the ghost of a prehistoric sorcerer to identify it as coming from a pre-human civilization, and sends the ghost back in time to learn how to translate it. It goes horribly right.
- In The Fellowship of the Ring, when Gandalf talks at the Council of Elrond about trying to find information about the One Ring in the ancient archive of Minas Tirith, he mentions how the language of Gondor has changed since the city was founded, and now even most of the lore masters cannot read the oldest documents in the archives. Naturally, that's where Gandalf finds the information that he needs.
- The Stormlight Archive: Scholars have untranslatable records of Dawnchant, the language of the planet Roshar's original occupants, the persecuted Parshendi species. Dalinar's visions force him to rant in Dawnchant while he experiences the corresponding Ghost Memories in his native language, making him a living Rosetta Stone when scholars transcribe and cross-reference them.
- Doctor Who: The Doctor has described Old High Gallifreyan as the lost language of the Time Lords. The TARDIS never translates written Gallifreyan, and River Song outright says that the language doesn't translate. During the Doctor's tenure as the Last of His Kind, he was the only person in the whole universe who knew how to read either form. River could also read and write the Old High form, and possibly Circular Gallifreyan (although she's never shown doing so). In any case, the Doctor doesn't use the language to communicate with other people, and even when the TARDIS uses a form of Gallifreyan writing on her viewscreens, it's always the Circular form instead.
- Iconian in Star Trek: The Next Generation is a lost language, the Iconians having been wiped out roughly 200,000 years prior to the episode "Contagion". The language is not known by any living being, though Data is able to approximate a rough translation by comparing Iconian glyphs to three other ancient languages (Dewan, Dinasian and Iccobar) believed to be derivatives of it. Very rough.
Data: This would appear to be manual override.
[presses a control and an Iconian gateway opens]
Data: That was not manual override.
- In Tales of Berseria, the ancient Avarost civilization had a language that emphasized emotion, with words having different meanings depending on the emotion of the writer or speaker. The language itself was lost to time when the civilization fell thousands of years ago, with only records of it left behind in ancient books. Since the language is no longer in use with the current populace of the Midgand empire, deciphering its meaning has proven to be difficult and takes a lot of time. This becomes important to the plot since deciphering the lost Avarost language is key to defeating Innominat.
- The Elder Scrolls series has several including Dwemeris (the language of the disappeared Dwemer or "Dwarves") and Falmeris (the language of the corrupted Falmer or "Snow Elves"). Finding a means to translate them is a plot point in Morrowind's Mages Guild questline and Skyrim's Thieves' Guild questline, respectively. There is also the old Akaviri language, which is only known by a few interpreters like Esbern, and it is even more difficult to translate because the Akaviri way of conveying information is couched heavily in allegory and mythical symbolism, which makes things problematic when they're writing down important information like a prophecy regarding the end of the world.
- God of War (PS4): The World Serpent speaks a language that no one else in the game speaks except Mimir, the wisest man alive. Mimir says everyone else who speaks the language has been systematically murdered by Odin and his sons.
- In Heaven's Vault, Aliya Elasra has to decypher Ancient, the ancient language of Nebula.
- Star Wars Legends: In Knights of the Old Republic, the Rakata language has long been lost to the Galaxy, since the Infinite Empire collapsed thousands of years ago and the civilizations that it had enslaved went to great lengths to erase any record of Rakata culture that they could find.
- Ancient Egyptian remains a partial example. Since the discovery of the Rosetta Stone, we now know what the words mean, but we aren't entirely sure how the ancients would have pronounced them. Just for one thing, the hieroglyphic writing system didn't represent vowels at all—we only know what consonants Egyptian words included.
- Latin is similarly an interesting partial case. We can read and write it, but nobody has any clue how the words were actually pronounced. While people can "speak" it, the pronunciations are all assumptions based on how said words would be pronounced based on the rules of current languages so nobody has any clue which words, if any, are being pronounced properly. For all we know we could be saying the equivalent of "Ow re-vore, mon sewer."
- Like their cousin Egyptian, several ancient Semitic languages were written in scripts that don't tell us what vowels their words included.
- The Byblos syllabary has still not been deciphered since its discovery in 1928. Several people have tried.
- Apart from isolated words, the few texts in the Etruscan language cannot be understood. The Romans actually wrote a Latin-Etruscan dictionary back in the day, but this text is lost.
- The Khitan scripts are likewise not deciphered beyond a few isolated words, mostly those related to the calendar.
- The Harappan script, used by the Indus Valley Civilization, has not been translated.
- The Hunnic language. Despite the Huns' prominence in Europe during the 5th century, nothing about it is known other than a couple of individual words. Even the infamous Attila the Hun's name is actually a Gothicized form of his true name, which also remains unknown.
- Another example is Linear A, which was used as the written form of the Minoan civilization's spoken language. To date, no Linear A texts have been deciphered.
- The last persons able to read rongorongo, a unique script developed by the Rapa Nui of Easter Island, died in the 1910s.
- The modern Romance languages include Spanish, French, Italian, etc. ... but at one time Romance languages were also spoken throughout central Europe, the Balkans, and north Africa. Various invasions and migrations replaced them, though, leaving virtually no written records behind.
- The Voynich Manuscript is one of the famous examples, other than that the book isn't complete, the writing is still not deciphered. It is thought to be a medical book for women on using plants and water to cure sickness, although it isn't confirmed.