Approaching this question from the protagonists point of view (as we are taking the role of good ol’ Henry after all), who is a native to the area.
As such, the Czech option is out of the question, as I would not understand it, even though I should (as Henry). Given that there is no option for German VOs in the poll, and given that even big budget games often feature questionable localization and/or voice acting, I am content with English.
Reasoning along the same lines, modern English with the odd archaic expression be it. Shakespearean English might be closer to the time period than modern, but it would not be more accurate from a historic point of view. And it would not feel natural. It might have been quite befitting a game like Ultima 7 (if indeed that was Shakespearean English – plenty of "thee"s and "thou"s there, at least), where a contemporary earthling gets magically summoned into a medieval fantasy world. But that’s not the case with KC:D. We are natives to the game world and thus what we hear there should come close to what we might hear in our birth place.
Mind you, when the choice is “International English”, I don’t want to hear Texan slang or something out of a Glasgow ghetto (anyone else remember the movie GamerZ?), but rather something that is devoid of any strong accent or regional flavour. So no Czech accents either. The only conceivable situation where some kind of an accent might actually make sence would be if we meet somebody in-game that isn’t a local (Dragon Age had that mostly correct, when citizens of Orlais speak with a french accent). But even then, not everyone has the same thick accent when speaking a foreign language … so it can be overdone.
Lastly, it is a matter of chosing the appropriate wording (and that may again depend very much on the NPC doing the speaking). Avoid slang and neologisms, but also make sure it comes over naturally, so that voice actors can easily pick up the mood and deliver a perfomance that does the situation justice. (Something that might be much harder, if the language is stilted and archaic).
Personally, I will probably turn on subtitles anyway and skip over bigger parts of the voice action, as reading is just so much faster than listening. But if it’s any good, I’ll certainly enjoy it more :-).