I've been reading up a lot of reviews that mention aliens speaking English in Halo. At first I thought these were placeholders for the earlier version of the game, since you can hear 'em in some preview movies. Then when I heard Cortana was uploaded into the Master Chief's suit, I thought maybe she activated some language translators that allowed the player to hear what the enemy is saying. Cortana probably studied their language carefully before she was uploaded.
Is there a real explanation described in the actual game? C'mon, gimmie some spoilers!