That's actually quite funny. First because I love this song and second because I would say dö dö dö instead of dun dun dun. "An onomatopoeia is a word that imitates a sound" Like omnomnom? Omnomatopoeia?
I would not suggest to learn it at the same time. They are similar but not that similar. And learning German should be hard enough. I guess Dutch has the same structure as German. Lots of words sound similar.
I did learn Russian? Good to know, was pretty easy, I barely noticed learning it. I had it in mind but somehow it never came to it. Can read some words though.
It's not even necessary to know more than your native language. And it's not even necessary to know that native language very well. But it can be useful.