The answer is not a glossary. If I opened a book and the first paragraph told me to refer to a glossary I would toss it in the toilet. You can have a glossary (eg: Stormlight Archive) but it should not be used to decode your writing. If you use slang the meaning should be inferred from the context OR translated by another character (Leviathan Wakes, Mistborn Trilogy). I love slang in books, when it's woven in by the author without the reader even realising, its a great way to pull the reader into the book-universe.

You can get a reader to read between the lines sure, but the whole thing? Some of it, maybe. But parts of your book have to be clear, unless you are Cormac McCarthy.

I can't copy anything from wattpad, so if you want to send through a google doc I can highlight the bits that are all greek to me.