The terms Fictional Language, Fictitious Language, Artificial Language, or Constructed Language in fiction has been used interchangeably in papers that I have read. Are there differences in these terms, and which is preferable?
All are correct, I think. But fictitious language seems like it would be a fake and meaningless babble with no consistent patterns. The best is fictional language—an example is J.R.R. Tolkien's new language in Lord of The Rings. Tolkien created a language for his characters of the elf species, with its own consistent vocabulary and grammer.
I agree with Ira. By 'constructed' we should understand any given language that has been artificially created by human beings. They can be applied to real life (e.g. Esperanto) or fiction, hence 'fictional' (Tolkien, G.R.R. Martin, Star Trek, etc.). I personally find 'fictitious' a less clear-cut term, as it can both characterise languages created for 'fictional' purposes and how they are perceived (as false or not genuine) inside a work of fiction by its narrative voice(s) or characters. I hope this can be of help.
Let me throw in another option: "fictive" - meaning feigned, created imaginatively, invented, i.e. a language which appears real but is in fact made up, not a 'natural' language.
Laurence Wright That would be a deeper layer in fiction. For instance, the language of J.R.R's Elves was fictional because it was invented by the writer J.R.R and was real and functional from the point of view of the Elves. Had it been that the language was just made up by the Elves, it would be fictive. Fictive is one layer of fiction deeper than fictional.