Eurish was the synthetic language invented by James Joyce for his last novel, Finnegans Wake. It contained forty or fifty other languages, including slang in all of them. The word "quark" originated in Eurish. In 2269, there were only a few hundred Humans who were fluent in it.
That year, Nyota Uhura advised James T. Kirk to have a message sent to Starfleet Command in Eurish so that the Klingons could not translate it. The risk was that it could take Starfleet Command some time to find an expert in it, providing they even recognized it. (TOS novel: Spock Must Die!)