Language Drift

A living language can change, sometime startlingly rapidly. In fiction it's employed to suggest cultural change, or a character's status as Fish Out of Temporal Water.

Compare Eternal English.

Examples:

In Gullivers 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 splinted into various successor languages. And the only ones speaking it are in the Catholic Church.
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.
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 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.
The 'lyrics' in Nie R'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.
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.
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.