Writers mainly use a Standard English that we can assume everyone literately educated learns, when it isn’t still their native tongue. Richelieu’s French Academy guards their tradition. In English, it’s quite enough to say that if Jane Austen didn’t use it, it’s suspect.
Use Standard rather than hip, now, TV slanguage because by the time it’s in print, it’s looking so five minutes ago. (Professional publication usually takes two years from deadline to book, not to menton all the time you’ll take to sell it). By the time return dates fill the inside pages of library copies, it looks definitely dated and, if you’re not careful, starting to become incomprehensible. Too soon the last check-out date is years ago.
One of the perils of writing mainstream YA fiction is you want kids to sound normal and today, which means in ten years the new YAs need a translation. On the other hand, good YA specfi, like Heinlein and Norton, avoided actual slanguage of their time for Standard with invented slang. Many YA publishers’ guidelines say “No science fiction or fantasy”: libraries don’t buy it because the old stuff is as popular as ever. But they always need new contemporary YA because kids can’t understand any longer a story where the protagonist doesn’t have a cellphone that takes pictures for evidence, or where they talk like 1995 did.
Adults are more forgiving of detail, but the story that’s now a period piece of dress and technology becomes foggy if you also used the slanguage and cliches of your time or, worse, your youth. Take Necromancer by Gordon Dickson. Someone says of the focus character, “He always could put the Indian sign on me.” Indian from India or redskins? Control or confusion or paralysis or adoration? Most of you have as little idea what that means as I do. This is played as a key character point, from one of the few to have known the person from youth, but it’s now totally lost to readers because the author used a (Western? rural?) slang phrase from his youth and put it in the future world.
Think how funny and opaque hippy slang stories can be, if you’re under forty. That’s any work stuffed with fashionable phraseology and cliches forty years later. If you want your grandchildren to collect royalty checks based on your work, think twice about not bothering to master Standard English, etc. It can be the difference between a short burst of sales and appreciation followed by limbo, and a long haul of royalties and reputation.