They are speaking what is called Old English or Anglo Saxon, the language one finds in Beowulf, for example. The language of that period was very different from English of today: it lacked the huge amount of Latin-based words that came in after the Norman invasion and there were later vowel shifts that changed pronunciation. Old English was different from the Old Norse that Ragnar is speaking (that language's closest relative today is Icelandic, which remains very similar to its 9th-century roots). But while Old English was different from Old Norse, the two shared enough for a speaker of one to learn the other with a certain amount of ease. It would certainly be easier for a speaker of Old English to learn to speak Old Norse than it is for a modern speaker of English to learn Old English. I learned Old English first and then learned Old Norse, which was made much easier for the background in Old English.