I have just rechecked the book and when they come across Cole's farm, Evanveline acknowledges that it looks like somewhere in the provinces. That is not conclusive since I imagine the provinces look pretty similar anywhere in the southern part of Thedas and since the land was burnt there would be no crops to indicate a definite area. Still I think it is likely to be a remote farmstead, probably somewhere in the remote part of the Heartlands but may be on the Dalelands side of Orlais.
He doesn't have to have a strong accent. I don't think that commoner in the Denerim market had a particularly strong accent. I think the strongest accents are the nobility and that may be a bit of affectation. In the Middle Ages there was no such thing as a standard accent. People had the accent of the region they came from. So in Elizabethan England, Walter Raleigh would have had a west country accent because that is where he came from. A noble from the north of England would have had that sort of accent. So someone from southern Orlais ought to sound different from the north or the east. After all, there is no standard America accent - it varies from state to state. Likewise in England, there is a fairly uniform accent among public school educated people but outside of that the variations are considerable.
It should also be remembered that he has been hanging out in the White Spire, overhearing conversations and might just as likely picked up an accent from there. Since the older mages could have come from anywhere, there is probably a fair bit of variety to choose from. Rhys has a bit of an Orlesian accent. That's how he was identified as foreign in Tevinter, but it may not be particularly strong in comparison to an Orlesian noble. Cole would most likely have tried to sound as much as possible like Rhys.