Knutti the Viking spoke a language called Old Norse. He would have called his language dansk tǫngu or norrænt mál.
Many of Knutti's customers at his store were the local Anglo-Saxons. They spoke Old English. Speakers of Old Norse and Old English could understand each other. This was likely a huge relief for Knutti!
With a little help, it is possible for English speakers to recognise many Old Norse words from a thousand years ago. There are many examples.
Can you learn a few words of Knutti's language?