Historical fiction is a balancing act–better metaphor, a tug of war, accuracy pulling hard on one side and accessibility on the other. If you make your characters talk like they actually did four or five centuries ago, you will quickly lose readers who don’t have the patience to mentally translate. For example, here’s a snippet of Meriwether Lewis’s diary from his expedition to find the Northwest Passage. He’s presumably writing and spelling more or less as he spoke:
…this may in some measure assist us to account for the heavy dues which are mor remarkable for their freequency and quantity than in any country I was ever in— they are so heavy the drops falling from the trees from about midknight untill sunrise gives you the eydea of a constant gentle rain.
And that’s just from 1803, and the man is speaking/writing English. If you’re trying to go farther back historically and farther afield geographically, it gets harder and harder to make your characters sound comprehensible.
On the other hand, if you make your characters talk like today’s teenagers, they just sound ludicrous. I knew I had gone too far in that direction when one of my writing group participants told me that my ninjas sounded a bit like her Yiddish-speaking relatives.
And the thing is, you don’t actually know if you’ve kept your balance until the thing is published and you get reactions from readers. That’s when you discover if accuracy had yanked you so far off into Meriwether-Lewis-land that nobody can understand you, or if accessibility has pulled you flat on your face.