A post I recently put up on the Audio Drama Talk forums on the subject of having characters in audio dramas speak in archaic ways to represent the setting of the story:
People have always used slang, and will always use it, so I see nothing wrong with putting characters speaking ancient (or non-English) tongues in modern English. The way I look at it is that the characters are speaking the equivalent way they would in their setting for who they are. So a nobleman speaks proper English, a streetkid speaks in a slangy way, etc. To me when listening to a period (or fantasy) piece, it’s about how the characters act that’s important.
Xena’s cast wasn’t annoying to me because they spoke modern English, it was annoying to me because they had 100% modern attitudes to go with that English. The language they used was fine, the way they used it was the problem.
I had to deal with this when I started to do my Little Gou adventures, and think how I was going to represent a bunch of people speaking Chinese (Chinese dialects even, not even proper Mandarin!) in English. I quickly gave up trying to simulate their language in any way, beyond peppering in a few key words and transliterated phrases, and stuck with making the rest of the details fit instead. It just wasn’t worth trying to confuse my audience for any kind of accuracy.