The writers need a new language. But they don't want to actually invent a new language. So they make the "new" language a cipher of English — usually, a substitution cipher — with the same words, grammar and all. Typically, an entirely new 26-letter alphabet is invented, but occasionally "cryptogram"-style ciphers are used, as in Final Fantasy X and Order of the Stick.
Works from [...] [tvtropes.org]