There are a number of guides to Shakespeare. The one I have is by Asimov.
Shakespeare is theater, so it's made to be listened to. Watching a good version of the play before reading can clear up a lot. I would guess that you can understand the vast majority of the English. There is a bit of slang from the time, plus, like many modern movies, there are various references that would be well understood by audiences of the time, but for the most part, once you get in the mindset, it's easy enough to understand.
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