Prolanguage
In conlinguistics, a prolanguage (Lat: prolinguāticum) is a pro-form that substitutes one language for another, usually in a work of fiction. The replaced language is called the antecedent of the prolanguage.
For example, in J. R. R. Tolkien's Lord of the Rings we hear about certain languages: Common Speech, Rohirric and the Hobbits' dialect of the Common Speech. Yet we only rarely glimpse the conlangs themselves, the most famous being Quenya, Sindarin and the Black Speech. When it comes to the Common Speech -- the language all the characters speak in during the course of the narrative -- we read their words in English (or whatever language the novels have been translated into). It is this use of, in this instance, the English language as a stand-in for the Common Speech that exemplifies the concept of the prolanguage.
As a literary device, examples abound. For example, many fantasy novels from Tolkien's Lord of the Rings to Howard's Conan to Jacques's Redwall series all use English as a prolanguage. Latin or Greek may also be used as prolanguages for the dead language of an ancient civilization. It might be noted that Jaques makes a very consistent use of dialects and accents of English to stand as prolanguages for the true speech of his animal characters. Only rarely do we see any devised languages or words, the language of the seals being a good example.
In translations of ancient Greek plays, sometimes standard English is a prolanguage for the dialect the play is written in, and Scots is a prolanguage for another Greek dialect used by certain characters.
In 'Allo 'Allo!, the prolanguage is English pronounced with various accents representing German, French or Italian.