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 zal kijken of ik hier nog iets van kan maken. Ik ga nu om half 3 aan de slag, dus verwacht eventueel een foutje :)