[dejj]

It's obvious: xkcd Voynich Manuscript.

https://xkcd.com/593/

[bee_rider]

Of course, it is a joke, but… RPG books are generally written to be comprehensible to the people running the game. Or at least, that is the goal.

[pavel_lishin]

That's the intent, but boy oh boy, is that not always the result.

[da_chicken]

Clearly you've never played Shadowrun.

[NL807]

I used to invent gibberish languages and writing as a kid.