[Jarshen]

I think the trick is to describe how the character speaks, slowly, with a drawl, breathy, excited and then choose the vocabulary like Wiglaf suggests and the syntax like Dragonlady.
You can always have another character commenting on their slightly archaic way of speaking.
I think accents are really hard to put down convincingy on the page, go with how you think your character speaks, ie if it is medieval what is that you want to suggest about him/her that makes medieval appropriate.