The different German languages and dialects. Germany(back then only as a cultural enitity) being a large power and a neighbour to Denmark naturally influenced the Danish languages up until the 20th century when English became more dominant. German was in the Scandinavian area the lingua franca, and the language of the elites and thus a large influence. The largest influence on the Norwegian language was Danish, since they often shared a king or Norway being a subject to Denmark. THis means that German played a role in the development of the Norweigan language. Also, there are two Norwegian languages. Bokmål and Nynorsk. Bokmål is the most commonly spoken and is a mix of old Norwegian and Danish. Nynorsk is a modern attempt to recreate the "true" Norwegian language.

It should however be noted that Danish and German already have a lot of similarities since they are both Germanic languages.

Follow-up questions are welcomed! I can recommend some literature aswell if you want.