an anachronism

an archaic word is a word that is no longer commonly used but is retained in a language because it preserves the flavor of a period.

Language anachronism

Language anachronisms in novels and films are quite common. They can be intentional or unintentional. Intentional anachronisms let us understand more readily a film set in the past. Language and pronunciation change so fast that most modern people (even many scholars) would not easily be able to understand a film with people speaking English as they did in the 17th century; thus, we willingly accept characters speaking an updated language. Unintentional anachronisms include putting modern slang and figures of speech into the mouths of characters from the past. Modern audiences want to understand George Washington when he speaks, but if he starts talking about "the bottom line" (a figure of speech that did not come into popular language until almost two centuries after Washington's time), that can be an unintentional anachronism.
A literary work such as Quo Vadis set in the time of Nero is written in Polish, a language that did not exist in Roman times and is usually translated into other languages that did not exist in ancient times because modern audiences generally do not understand Latin, the language of Rome, any more than non-Poles can be expected to understand Polish. That sort of anachronism is generally excused.
At the most blatant, linguistic anachronisms can demonstrate the fraudulence of a document purportedly from an earlier time. The use of terminology from 19th and 20th century antisemites demonstrates that the supposed "Franklin Prophecy" is a forgery, as Benjamin Franklin died in 1790