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 :) 
Ik zou gewoon even kijken per situatie en kijken of daar een komma nodig is of niet; niet op elke plek past zo'n Oxfordkomma even goed. Hier zal ik de komma wel weer terugzetten.