Interesting that it's been a thousand years and yet they're still speaking approximately the same language with absolutely no difficulties understanding each other.
Fun fact, a native English speaker today would not be able to understand Old English from the 11th century, as William the Bastard had not yet conquered England and made French the official language. Not only that, over the past thousand years there have been several great shifts in vowel pronunciations, not to mention dialects and accents. A thousand years before the 11th century, and the nordic influence would not have entered English, since the Nords didn't invade until around the 8th century.
A thousand years is a long time. I'd expect these individuals to not quite understand each other, though they might be able to communicate basic concepts if they tried hard enough.