HELLO. I was a Medieval Literature major in college, so if you just give me a few minutes I can figure out what Merlin and Arthur would have spoken.

The main thing to remember is that Arthurian canon is crazybugnuts and actually spans a few hundred years, and there are points in time when it's a French myth. So to start out: they probably would have spoken a nasty weird hybrid of Middle English (eg, Chaucerian english) and Classical French.

Uhhh I'mma look up some samples and you can pick (I assume you don't want classical french V: )