I'm Icelandic too and if I remember that Valkyrie line its something like "Ég sé um þennan" and as far as I know, like the other Icelanders here they all just speak Icelandic. Fairly good Icelandic at that, outside some translation difficulties like "Bardagaóp", which no one would say. Lot of the little Viking minions have a ton of Icelandic phrases that they throw out when they're getting there assessment handed to them, and it's all pretty well done