Dutch translation improvements

DO NOT MERGE YET! STILL WIP!

I'm looking at the Dutch translations and see if I can fix or/and improve some translations. I will look further today at this, so till I say it, do not merge it yet. In the mean time the other Dutch translators (for example @Gymnasiast ) can look at the changes I have made so far and can descide if it's a good change or not. Just want to let you know I'm working on it, so we don't do dubble work :) 
Zelf geef ik ook de voorkeur aan de Oxford-komma; zoals @Gymnasiast al zei voegt het een logische (a.h.w. hoorbare) pauze toe aan de zin. Wat mij betreft houden we ze er dus in.