The dilemma of the author of exactly how much jargon to put into a fic. Naturally, the sort of people who read fic will be the kind to catch a really funny or subtle reference, but you also run the risk of making it completely opaque to someone else. Fics that try to maintain a sort of Original Flavour usually avoid the equivalent of language too 'informal' for it.
In other cases, language is another problem. This almost universally applies to anime Fanfic, where every writer has to debate exactly how much of a non-English language should be used until it needs to be footnoted just in case, and ultimately begins to read really awkwardly. As a rule of thumb, using random words is frowned upon but Honorifics and show-specific terminology are perfectly fair game. The grey area comes into play with common terms that may not be fully translatable, have a certain culture connotation or just sound strange in modern speech. Occasionally, the translated phrase is linguistically correct but sounds odd enough it becomes a kind of jargon, because the audience would never use it aside of the specific connotation.