I don't know about the Dutch but apparently the Flemish don't understand German without having learned it at school.

I speak both some German and some Dutch (as nth languages, I can understand them fine but speaking is hit and miss) and sometimes I don't notice which is which and answer in the wrong language, to me they're almost the same language with a different accent. I translate the German into some Frenglish mess for my Flemish friends to help them understand and it works great.