I think you limit your options too much here. There are many ways to deal with language. First of all, there is no modern English to begin with. Many people belonging to the same linguistic group speak differently, they have variations in accent, but also in vocabulary, grammar, tone and discourse. All these things make up the ‘register’ and there you can have a ton of registers for the same language.
Characterisation can thus happen based on language. But do mind that registers are not random. For instance, if I would just render the following sentence: “Oy, what yu’re doin’”, you would immediatly think of a kind of Manchester working class accent. Such a phrase tells something about my identity, my social position and the cultural practices.
In the Middle Ages, this was very much the same. Even though I am not an expert on Bohemian history (my personal work as a historian focuses on the High Medieval Low Courntries), I am quite sure that it is not going to be too different. For instance, you have a German speaking upper class. Noble families that moved in from the Realm. You expect those men, certainly in the early fifteenth century, to speak in a ‘courtly fashion’. They would have learned the refinements of courtly interactions from a very young age (even the most ‘blokiest’ of knights). Such men would often verse more complicated words, just to show off there education and well-versed manners. You could use a typical South-British accent for this type of people. Clerics, having received a scholastic education, would of course top that with nice diplomatic even semanticly nice wording (do not forget that retorics were a part of the trivium education). A little over the top PE English (think Oxford dictionary) would be excellent for that type of people.
Of course, you would have a more rural class as well. Using simple phrases, simple words, straight to the point. IN many series Northern or industrial city accents such as that of Liverpool or Manchester are often used. Another example can be found in the English of Samwise Gamgee in Lord of the Rings (Tolkien by the way is an expert in using different registers troughout his book and do mind that not all his language is archaic).
For the authentic Czech nobility or lower knighthood you can use for instance Welsh, Irish or Scottisch speach (but subtly, otherwise it becomes a farce). Certainly Welsh is a great example. THe term Welsh is etymologically derived from ‘the other’ (little historical side note, the country I come from, Belgium, has a Dutch and French speaking part and the Dutch have called the French speakers ‘Walen’, which is related to the word Welsh, also to point them out as the ‘other speakers’). In Bohemia, you actually had the same thing going on, being a frontier area with the Slavonic language group.
Now there is much more to be said about this topic, but I hope this might already serve as an inspiration