Which plaintext languages to try?

[ReneZ]

I don't know whether many manuscripts were created in Czech in the early XV century.

Yes there were.

However, this level of language distinction (Czech vs. German vs. Latin) is not relevant.
They all behave in similar ways (20+ alphabetic symbols, vowels and consonants) which does not match Voynichese at all.
Whatever transform can convert Voynichese to any of these languages, will work equally well for all of them.

I have used the following comparison before, but probably not many people have seen it.
Converting Voynichese to a known language is like travelling trom Australia (Voynichese) to a city in the UK.
Your trip does not depend a lot on whether you want to go to Wigan (Czech), to Watford (German) or to Hartlepool (Latin).
You first need to figure out how to travel from Australia to London. The rest is details.

More importantly even, I would strongly advise to include all arabic languages and Hebrew in one's consideration. These have barely been taken into account until now, so why ignore such relatively unexplored areas.