You need to distinguish between motifs and developed narratives that contain several motifs. Once you have 2 narratives that contain 2 or more of the same motifs you start to think a linear connection likely.

There are all sorts of ways stories can move from one language to another in relatively short periods of time. The most obvious is perhaps direct translation by sophisticated bilingual people. Another is trading nations such as Britain and Scandinavia over long periods of time. In fairly recent times it's said that East Yorkshire dialect had so much in common with Danish that East Riding farmers meeting the Danish fishermen in the inn could hold a conversation.