This question's come up before and I wish I could remember where.  I can say with maybe 90% certainty it isn't Dovahzul.  Listening closely, it's hard to pick out anything intelligible.  The lyrics probably don't mean anything much like the vocals in Unbroken Road or Frostfall.  My personal opinion is that if Bethesda had gone through the effort of writing actual lyrics for the song, they would have released the lyrics much like they did for the main theme.

Yeah, I asked this as well. They should have at least made the lyrics actual words.

It might be old Nordic.

To me it sounds like chanted gibberish so I cant give you a useful answer. I'm no expert but it does'nt sound like Dovahzul.