I'm not sure how it works in the German/Dutch Corps.
Both Germans and Dutch commonly know English, officers are simply expected to know English and most Dutch understand German because
a) the languages are somewhat similar (Dutch is in between English and German)
b) Dutch people watch German TV stations because they don't have a good choice of Dutch stations
c) they can/do learn it at school

As a German, there's a golden rule, though:
Never initiate a conversation with Dutchmen in German. Begin in English, eventually they'll propose to talk in German unless they're really much better in English than German.
It's about a "small neighbour" syndrome and a bit also about the "the stole my Grandpa's bike" problem.