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But using language of the time period would be extreme. Anyone try reading Shakespeare? While a classic, not many people enjoy it for a light read.
While I do enjoy Shakespeare as a bit of light reading, anything I read now had better be in modern English. I get footnotes and scene notes in Shakespeare and I expect it to take a bit longer to read than normal. (Though I've read and reread the plays so often I don't really need the footnotes anymore). If an author does provide that kind of thing then it had better be amusing a la Terry Pratchett's asides within asides or Susanna Clarke's world building notes/snark. These are just personal preferences, mind you.
I guess my main point is you expect to read a story that's written in the style of language that is currently spoken. When you make it sound authentic to that time period it reads as gimmicky.
Another thing to think about is "is this the clearest way of saying something or will I lose the reader?" If the goal is a very educated student of languages, go right ahead with the antiquated language. If it's not, pick up a non-modernized/translated version of the Canterbury tales or Beowulf. The sentence structure, the passivity, the unclear pronoun usage all combine to make it more difficult to understand. Both of these stories have beautiful language, but it is very dense. I only made it through the Canterbury Tales by reading a text that had the original text juxtaposed against a modern translation.