The Viking are sea pirates!
I see! Didn't they speak the Norse language?
What's the Norse language? What country speaks such?
Old Norse
Old Norse was a North Germanic language that was spoken by inhabitants of Scandinavia and their overseas settlements from about the 9th to the 13th centuries.
The Proto-Norse language developed into Old Norse by the 8th century, and Old Norse began to develop into the modern North Germanic languages in the mid- to late 14th century, ending the language phase known as Old Norse. These dates, however, are not absolute, since written Old Norse is found well into the 15th century.
Old Norse was divided
to approximately  200–300. While there are remains of Viking runestones from the Viking Age today they are rare, and vary in use of orthography depending on when they were created. Rune stones created near the end of the Viking Age tend to have a greater influence from Old English runes.
An understanding of the writing system of Old Norse is crucial for fully understanding the Old Norse language. Studies of remaining rune stones from the Viking Age reveal many nuances about the spoken language, such as the constant use of alliteration.