I think the intros would be best without any audible narration, unless someone has enough balls to pronounce them in the faction's native language. It would be a little weird that the only English you'd hear in game were in the intros. The only English voices in EB at the moment are the campaign map ones, the pre-battle speeches and the Greek and eastern "Run to deh hillz!", which will most likely be phased out in the future. Just a simple text narration would be the best and easiest in my mind.

An out of topic question for all the modders out there:
Spoiler Alert, click show to read: 
How do I disable the vanilla Greek and eastern fleeing voices, ie. "Run away!", "Run to deh hillz!"? I've become so fed up with them that I tried muting all voices, but they are apparently considered "sound effects", so I've muted all sounds and am listening to random music from foobar instead when I play RTW. I tried it on my own and it lead to nothing, so someone here probably knows better.