From ToM:
Those with the heads of wolves were often among the more intellingent; some of the Saldaeans claimed to have heard them speaking the human language to bargain with or trick their opponents
And of course, Narg was a wolf-headed Trolloc that tried to trick Rand.
As for where he picked it up, well all the Myrddraal can speak the common tongue so they must learn it from somewhere. There are humans in the Blight, after all. There's the Aiel Town, and Demandred tells of efforts to breed resistant crops in the Blight in the LoC prologue, so likely other Towns as well.