Why did Old Scandinavian speakers consider their language a single language?

During the Viking Age (c. 750–1050), speakers of Old Scandinavian considered their language to be a single entity—sometimes referred to as "Danish" in contrast to "German"—primarily because they could communicate with one another without difficulty despite regional variations.1

This linguistic unity persisted throughout the period of Nordic expansion, during which Scandinavian speech was established across a wide geographic range, including Iceland, the Faroe Islands, and parts of the British Isles and Russia. While the differing political orientations of various kingdoms eventually fostered dialectal differences—such as the distinction between the more conservative West Scandinavian area and the more innovative East Scandinavian area—these were not yet viewed as separate languages. It was only as kingdoms became more centralized and distinct writing traditions emerged in the later Middle Ages that the perception of a single, unified language began to fade.12