Your poll lacks a proper amount of options.
I fully agree with you that having Shakespearean English does not make any more sense than any other language, except the language that was used by the people living there at the time of course. Thus, using a modern English would be preferable since then the player will easily understand that it’s only supposed to be a translation, not how they actually would talk. However, I can not for the life of me see how US or “International English” as you say, whatever that may be (Yes I have heard the term before yet no one has ever succeded with telling me exactly what it is. People from different places will speak with different accents, there are no universal dialect), would be better than using English accents. I assume that what you mean with “the language everyone around the world understands” is that most people in the western world are used to hearing American accents on the television and other forms of media and entertainment, however, I’d say this is actually a bad thing. Since we’re so used to hearing it in modern contexts, it would just be off-putting and break the immersion hearing it in a medieval environment. English accents, however, are not as commonly heard, mostly due to the fact that England only has a sixth of the amount population as the US, and we’re a lot more used to hearing it in a medieval setting, making it feel a lot more natural in the world the game is set in. It also sounds a lot more beautiful. An example of this is Game of Thrones where they use British accents but a rather modern vocabulary, and it works perfectly. Now imagine if they used American accents. Is it just me or does the thought of that makes you want to attend a Westerosi wedding as well?
On another note, while I don’t mind you writing the story in Czech, please hire a good, professional, native English speaking writer to write ALL of the dialogues. I would be incredibly disappointed if the dialogues are as terrible in this game as they were in Skyrim, a game which you claim to hate. A rich and beautiful language, that still feels natural, is required for me to fully enjoy a story driven game, with a few exceptions. It’s, of course, very important to hire talented voice actors, which I’m very much expecting considering it was one of your stretch goals.
Summary: You should’ve included an “I prefer modern English with English accents, but with some archaic words and phrases” option since it’s the best way to go. Don’t underestimate the importance of writing good dialogues or having good voice actors. All in all, if your Story, dialogues and voice acting come even close to the brilliance that is Assassin’s Creed IV: Black Flag, your game will be amazing (assuming you don’t fuck up the world design and gameplay).