I'm German, I don't speak Dutch. But I was able to follow a Dutch tour guide in Den Haag just fine when she was explaining things in Dutch. She kindly repeated everything in English for my benefit (I was the only foreigner) even though I told her I understood her just fine in Dutch.

You have to "adjust your ears" a bit but I think if you know German and English then you can understand Dutch just fine if it's not slang.

It also depends on the particular dialect a German speaks. Dutch is effectively old German before all the various alterations and "reforms" to the German language that were instituted to create fragmentation between the germanic people of Europe, i.e., English, Dutch, Germans, Austrians, Swiss, Belgians throughout the ~16th-20th century by aristocrats driving wedges between peasants between kingdoms and dukedoms in order to define their own nations/ethnicities through language and culture so their royal families could rule over and would find it difficult to associate with each other. It is one of the things that also contributed to the fragmentation of Germany before unification, language barriers that even created unique cultures between sides of a valley that were in different dukedoms.

A similar thing has caused the tension between the germanic and Romance languages that followed the Roman border line N to S that separates Europe.