Could incantations explain some of the problems of the Voynich Manuscript

[JoJo_Jost]

But how do we know that?  From documents written in "Bavarian"?  They may not reflect the spoken language at all.  See French...

Yes, you can tell from the written language. In the 19th century, many teachers wrote down their regional dialects and thus preserved them for posterity; that was a trend at the time. This alone allows us to recognise developments.

But of course, we don't have any sound recordings from the 15th century . However, since there were no complex spelling rules in the 14th/15th century, many words were written phonetically (not purely phonetically, but you can recognise – or guess – the different language variants).