Which plaintext languages to try?

[Rafal]

A medieval writer writing in Czech would most probably write "cz" instead of "č".
Guess why this language is called "Čeština" by itself and "Czech" everywhere else?  
And in Polish language which is similar to Czech you use "cz".

Are there any known examples of medieval Czech ciphertexts?

Personally I don't know such texts. But they would probably be the same like everywhere, just maybe simpler than Italian ciphers which were cutting-edge technology for a long time.

Here you have a text about Hungarian ciphers which is a neighbour country to Czechia. I think you may expect the same level of ciphers in Czechia:
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