The Modern Forgery Hypothesis

[Jorge_Stolfi]

no one writing in Latin would have written such a letter.

This is not true.  People had to use Latin for certain correspondence, just like today people have to use English on certain forums.  Whether they were fluent like Cicero, or could only scratch a macaronic pastiche.  

Barschius was Czech and not a priest.  His Latin could be arbitrarily bad.

All the best, --stolfi