Award winning author Hilary Mantel writes an article about how to walk the fine line between authentic dialogue in historically-based fiction and being able to engage your reader with clear language use. She uses Tudor English as an example.
A writer must broker a compromise between then and now, and choose a plain style that can be adapted to different characters: not just to their ages and personalities and intelligence level, but to their place in life. I use modern English but shift it sideways a little, so that there are some unusual words, some Tudor rhythms, a suggestion of otherness.
I studied Elizabethan literature at university, and there are certainly unintended confusions and deliberate puns that either make no sense to the modern reader, or not enough. 'Quaint', for example, was pronounced in a way that made it easy to pun on a vulgar word for female genitals which rhymes with 'hunt'.
Hilary gives her own example:
The verb "let" for example, now means "permit"; to the Tudors it meant "forbid." What we call a clever man, they called a "witty" man.
Definitely worth a read if you base or intend to base your dialogue on historical languages: http://online.wsj.com/article/SB10001424052702303459004577363870847167262.html