In the The Eagle's "making of" somebody said they decided to have the Romans speak with an American accent as teh Roman Empire basically held a similar cultural sway over the (then known) world as the US do over our modern world. Actually I found that logic not too bad. They didn't promote the idea that that is good or bad, just stating a fact to find some lines of inner logic. Worked for me.
Still, the movie is not going to be one of my favourites. I didn't like the Seal people nor did the acting look too convincing to me.