Hi, I thought about this a lot lately - as with more aspects of this game I guess it is hard to find the right balance between historical accuracy and contemporary entertainment. Recently I came across a good article about this illustrated by the differences between two TV series about Borgia family (here)
The argument is that in order to make historical settings understandable and enjoyable for modern audience, you need to adjust a lot of things. So I wound not see as problem to have characters speak in more contemporary language (words) if they use the language in historically accurate way - how they addressed they superiors, that there were great class differences in the ways of speech a lot of references to religion and god as basically everyone was believing christian and so on… also it should reflect completely different morality values to modern day - common use of violence, superstitions, fear of devil and hell, conflict between cruel reality and idea of good christian life. I think it is more important about WHAT and in WHICH MANNER are people in the game talking than language itself.
Also considering that in that age there were multiple ethnicities living side by side in the Bohemian kingdom and upper classes commonly knew more foreign languages and Latin as well, it was very common that people understood and spoke Czech and German without the need of translation or they used some broken version when not speaking in they native language. For example I can imagine someone to talk to they landlord in Czech and lord replying to them in German, while both characters understand each other (something like today’s Czechs and Slovaks do). This aspect I can see as more important for players immersion than use of anachronisms.
This could be interpreted with understandable (thought not necessarily most contemporary) version of main characters language (either Czech or English) and appropriate foreign languages (also understandable for those who can speak it) used by other characters with subtitles (like in the example of “With fire and sword” above).
So I have no problem with main character speaking English (Though I am Czech myself) - it is more question of world-wide accessibility of the game, than gameplay itself in my opinion. But I would appreciate if you could use a language version that would not sound too cheesy or forcefully “archaic” in the end. As it often happens with speakers on medieval fairs who use “old sounding words” to support atmosphere of event, but cannot construct sentences in old fashioned ways so it ends up as ridiculous nonsense. After all in 15th century they spoke in the way appropriate for they time, not in some archaic manner. So if you want to bring attention to what is actually happening in the game, you should say it in the language that is appropriate and understandable for the player and deliver experience based on periodical settings of story and behavior of characters…
(sorry for the long post)