It’s also not like we’re told the federation has had complete access to everything about the society. There could be more traditional stories shared with their children as a basis for the language, but unshared with the federation.
All language lives within a culture, and words only convey meaning through a mutually shared understanding. They’ve made allegories first class parts of language, the equivalent to our individual words. Similars happened in programming in so called third/fourth generation languages.