(Middle) Low German (MND: Mittel-Nieder Deutsch) from 13/14th to 17th century would be the closest parallel with such concept as 'lingua Franca' in the Northern Seas. The penetration of the German Hansestic merchants across the cities in the North and Baltic Seas must have played an important role in making this dialect of German so popular.
The northern lands except England thus used related Low German dialects, a development faciliated by the reach of the Hanse, which made Low German the language in German offices from Bruges to Riga. Perhaps with rhetorical exaggeration the hegemony of Low German has been compared to the influence of French in the Seventeenth and eighteenth centuries (Nicholas 2009: 191).
All the three Nordic languages (Danish, Swedish, and Norwegian) got profound change from Low German, used by the visiting German merchants in their cities like Stockholm and Bergen after the Black Death. The ability of commanding good Low German was also employed as criteria for admission into German Hanseatic Office (Kontor) as a member of their community (Selzer 2010: 84f.). So, Low German was primarily a language for conducting business with such German merchants.
After the arrival of printing press in Northern Germany in the last quarter of the 15th century, however, printers employed Low German as the language of the printed books, and such early printed books and its contents were also widely distributed in Low German across the Northern Seas in addition to Hanseatic merchants and their goods. I'll conclude this brief post by showing an interesting example of such cultural interaction across Northern Europe mediated by Low German: A literary analysis of M. Kalinke on the Book of Reykjaholar, the Icelandic manuscript of the compilation of the saints' lives and allegedly compiled in ca. 1520 in Iceland, reveals that the 16th century Icelandic scribe translated some legendaries of the saints from Low German early printed books and incorporated them into the compilation (Kalinke 1996). Hanseatic merchants those who had taken a visit far in Iceland since the end of the 15th century [Added]: (Friedland 1984) also left their trace in the parchments of this very late medieval Icelandic manuscript.
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