I Was talking to a friend who writes regularly, but doesn’t post it anywhere. He is safe behind his walls as an unknown writer. His work is good. The imagination he displays, would put most writers I know to shame. There is only one thing I can see in his writing that needs work, confidence. He chooses safe phrasing, doesn’t stray outside the sandbox of words his computer says are correct. I realized how much this translated from his personality, to his writing when he was reviewing something for me.
Him: What is Schmraugh? That isn’t a word is it?
Me: My character sure seemed to think it was.
Him: How does the reader know that though?
Me: If you drop something heavy on your foot and you yell something in Russian, I am pretty sure everyone would get the point.
Him: Yeah, it just wouldn’t leave my spell checker, so I thought I would ask.
His work just needs that punch. Not necessarily made up words, but a more open approach to how people communicate. Slang words may be difficult, but if you listen close to the frame of reference, you will understand the context. It can really add to a characters individuality. It can even be used as an identifier as to who is talking without having to end every line with “He said jokingly.”
It is possible to take this too far and make your character unintelligible, but generally a specific set of made up words won’t hurt the readability.