Language Drift

Language Drift is the process by which languages change and evolve over centuries or even decades. Accents, dialects, vocabulary, even basic pronunciation can undergo dramatic shifts in as little as a few decades, and when the span is longer the changes can be even more pronounced. Add on top of that the changes in slang, memes and pop-cultural references, and two individuals separated in time by as little as a century can be almost unintelligible to each other despite allegedly speaking the same tongue.

A good example would be the language you're reading in right now. English has undergone a profound evolution over the past fifteen centuries and more. In very broad strokes its history goes something like this: It started out as "Old English", a dialect of Anglo-Saxon (the tongue that would eventually become German) spoken by Viking invaders and settlers in Great Britain, probably best known as the language in which Beowulf was written. Bits and pieces of the original native British languages got welded onto it, along with stray parts of Latin that didn't run away fast enough. In the wake of the Norman Conquest it then absorbed far more French than was probably healthy, becoming in the process "Middle English", the language of Chaucer and The Canterbury Tales.

English continued stewing for several centuries, slowly mutating and evolving, until the Great Vowel Shift began circa 1400. Over the next three hundred years or so, almost everything about English pronunciation changed -- while at the same time standardization of English spelling began. (Since the two were completely unconnected to each other, the result was modern spelling, which sometimes makes absolutely no sense unless you know how things were pronounced in the 13th century.) Right in the middle of this period was the Elizabethan era during which William Shakespeare wrote; it had mostly modern-looking spelling, but its pronunciation still owed quite a bit to Middle English. It wouldn't be until around 1700 that the Shift concluded, leaving us with Modern English (which had two noticeable variants -- British and American English).

And that's just what English sounds and looks like -- but language is more than that. It also includes and is affected by things like pop culture, slang, memes, "common knowledge", running gags, in-jokes, politics, social priorities, technology, subcultures and other elements and influences beyond the mere mechanics of the language. Let's step back to Shakespeare for a moment as an example: despite Elizabethan English's superficial resemblance on paper to Modern English, modern readers find it difficult to understand the surface level of his works without glosses that provide missing context, let alone understand all the jokes and puns they're littered with. The puns in particular are almost impossible to understand without help because after he wrote, pronunciations kept changing. Words that sounded alike to him and his audience don't sound anything alike today, puzzling generations of students. And the slang his characters used has been supplanted several times over, rendering more than half of the rest of his jokes incomprehensible.

Usage and vocabulary are also constantly evolving, the latter sometimes incredibly fast and disruptively. Words stop meaning what they used to mean and get repurposed over time. A good example is the word "wand". It used to mean "slender, flexible stick". It now means "magical prop", a shift in meaning that took place centuries ago. And if we really wanted to belabor the point, we could get into the finer-grained changes, like the evolution of grammar -- for example, the divergence between "more so" and "moreso".

The point here is, language is always changing -- changing what it sounds like, how it works, and what it can discuss. Despite technologies like TV, radio and the printing press having a slowing effect on that change, it has and is never going to stop. Just think about the difference between English in 1980 and English today. Most pronunciation is the same, but there are words we use every day for concepts that would be utterly alien to a person from 1980. Now imagine it in the other direction... what will the English of the late 21st century sound like?

The skilled and knowledgeable writer understands this and employs it when appropriate for simple verisimilitude, to suggest or emphasize cultural change, or to highlight a character's status as Fish Out of Temporal Water. Hack and amateur writers, on the other hand, ignore, forget or never knew this. They may write characters from the tenth century as thought they just stepped off a street corner in Liverpool or Philadelphia, or resort to generic "old-fashioned"-sounding language. This runs the risk of breaking the audience's Willing Suspension of Disbelief.

Contrast Eternal English, or for linguistic change altering how a work is later perceived, see Have a Gay Old Time. See Ye Olde Butcherede Englishe for when the author knows just enough to get it wrong.

Examples of Language Drift include:

Film

In Idiocracy, the massive proliferation of stupidity in American society has resulted in a corresponding degradation of the English language. The protagonist, our Fish Out of Temporal Water, is regarded as sounding "totally 'tarded 'n shit" when speaking normal 21st-century English.

Literature

In Gulliver's Travels, it is stated that most Struldbrugs are incapable of speaking more than a few words to those around them due to that trope. It is unclear how much that trope affects the written language, since there they suffer another problem - they can't remember what they just read.
A major theme in Riddley Walker. It's post-apocalyptic fiction, and the book is just barely understandable, if you read it carefully and sound it out phonetically. Their conflation of various words of today's English (notably "Adam" and "atom") lead to much of the background, folklore, and plot.
A Canticle for Leibowitz. By the time the events of the novel take place, English had long splintered into various successor languages. And the only ones speaking it are in the Catholic Church.
In Michael Crichton's Timeline, three characters who travel back in time to The Middle Ages have to learn how the French of that time differs from modern French. Even the character who already knows the written language of 1357 has to learn how it's pronounced and inflected.
In the Crest of the Stars novels, the language of Jinto's home planet of Martine is said to be descended from English but when they hunt down someone who actually does speak English Jinto can't understand a word of it. The Japanese = Baronh and English = Martine in the anime is presumably a Translation Convention.
Used by Isaac Asimov in his Empire and Foundation series. Pebble in the Sky features a protagonist who inadvertently steps into the future, where his 20th century English is unintelligible to all except a few historical linguists. Even they struggle. In Foundation, Asimov repeatedly refers to the standard Galactic tongue as evolving throughout out time, and isolated worlds tend to fall behind, resulting in Ye Olde Butcherede Galacticke Standarde. The change, however, is noticeably slower than in real life - it takes about five centuries for a document to start sounding queer, and a historian states the difference between his language and today's English is not that radical - different pronunciation and a lot of obsolete words, but not that different in principle. However, an isolated planet had its language completely unchanged, because, apparently, its people depend on robots, and maintaining the same language (in a society with little personal interaction) is easier than changing the programming.
In the Legacy Trilogy by William H. Keith Jr. (writing as Ian Douglass), due to relativistic travel, characters come back to Earth after many years away and find that they're unable to understand what people are saying or be understood themselves without special translation software.
In The Forever War, by the mid-21st century, pronouns have already begun to shift. Centuries later, 20th century English has become the Lingua Franca of the Force, since most of the military brass, having lived hundreds of years through relativistic travel, speak it.

Video Games

The 'lyrics' in NieR‍'‍s soundtrack are written in futuristic versions of French, English, Japanese, etc. (and despite sounding like gibberish, you can actually tell which language they're been based upon), because the game itself takes place a few thousand years after the 2000s.

Western Animation

The Business Men from Adventure Time have very bizarre grammar, but when one combines the Word of God statement that the show takes place after a nuclear war wiped out most of humanity and the fact that they were implied to be Human Popsicles from the past, it becomes apparent that they are speaking contemporary English; the other characters just have Translation Convention on their side.
- When Finn found the tribe of Humans they're speaking a language that could hardly be recognized as English.

Real Life

The archaic Latin chants of the Roman priesthood were indecipherable even to Cicero in the 1st century BCE. The only recognizable words are Ceres, Janus, and thunder. The Donation of Constantine was recognized as a forgery when it used 8th century CE Latin words in a document supposedly written in the 4th century CE. See the wikipedia article on Latin for a history on the different forms of the language.