The reason we can’t ‘translate’ it is because the author was just writing gibberish around the pictures, to impress the ladies who listened to his fanciful tales about what he had drawn and written in the book. It is very likely none of them could read anyway.
It's not that easy; first he'd need to invent a completely new alphabet; then he'd need to invent a language that is written in that alphabet, with grammar and other rules. Only then one can "write" something that convincingly resembles a text.
There is another catch. If he is caught with this book, the Inquisition would ask him, quite insistently, to reveal the language, so that they can translate it and see in what exactly words he is expressing his love for the Devil. The inquisitors wouldn't take "no" for an answer, and they wouldn't believe that the book is a prank. In their opinion (quite justified, IMO) very few people would go to such a great length to make a practical joke.
to impress the ladies who listened to his fanciful tales about what he had drawn and written in the book. It is very likely none of them could read anyway.
As I understand, ladies 500 years ago weren't too demanding. Besides, they'd be bored by the book in an instant - like if I give you a large crystal of salt and start blabbing for hours about its physical properties and X-ray crystallography (especially if we both know nothing about that.) A book would be of no interest to illiterate ladies, beyond a few pictures - and there aren't too many, and majority of them are [unknown] plants.
One theory is that the book was made as a con job, to be sold to a collector for good money. The text then is indeed meaningless, but some effort was put into making a language.