I am writing a fantasy novel and I wanted a few of the characters who speak a secret language. I didn't want to create an entire language from scratch. So I started looking for real secret languages that were no longer in use. I found Polari, and I really liked it. I not only liked the words, but also the clipped pace, and the sense of humor in it. Since it had smatterings of Yiddish and several different European languages, it sounded more exotic than back or rhyming slang.
The problem is, while its no longer fluently spoken, it was in use fairly recently. Even if I took out all of the crossover words that survived and became modern gay slang, I still think it might sound familiar enough that some readers would immediately recognize the brief conversations as Polari. If it were very few, maybe it would be sort of an Easter egg that would make a few people snicker. But if it's still fairly well known, it might make the characters stand out as badly as Jar Jar Binks did when he said, "Exsqueeze me?"
I would have fun being a little bit creative and make up some terms, but I'm no Tolkien. I'd like to just add things that don't exist in the real world and take out phrases and words that are more obvious in origin.
Anyway, I'm asking because I'm American, and I'd never even heard of Polari. But in the UK, it might still be "funny, but not not-funny ha ha." If you catch my meaning. I didn't even know back or rhyming slang existed and they are still used!