In the movie, the Arab (Antonio Banderes) does not know the language of the Norsemen. He learns to speak their language by watching them. The first problem is that it appears he manages to learn their language in just a few days. That is pretty incredible. Even if that were possible, there would be one other problem. This is the fact that it's just a group of men alone. It would be almost impossible to learn the language without some kind of reference points. I could see him learning some simpler words like fire, horse, food. Things that he could observe them talking about and interacting with, but not becoming fluent in the language and certainly not so quickly.