Many master's programmes are taught in English. Most bachelor programmes at the Icelandic-language universities are taught in Icelandic and ask you to prove you can follow them. That single fact decides which courses are open to you, so check the language of teaching, and the entry requirements, on the exact course page before you apply. Our education page explains how a foreign diploma is turned into Icelandic credits.
Editor's note. One email to the admissions office of the exact course you want, asking for the full list of entry requirements including language, is worth more than a month of reading. The answer usually takes about a week.