Two things

1. If you’re able to encrypt; then done case closed.
2. If you’re not able to encrypt (e.g. shared system or cameras watching you type, or written documents) then then slang can actually be actually effective and easier than a whole language. It can be dynamic and fluid, and gives the author the defense of metaphors and misunderandings. It’s basically the only way to have hidden communication with others “in plain sight”.