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 slaap 's nachts ook wel hoor @Gymnasiast . Maar als ik vakantie of weekend heb, loopt het nog wel eens uit. Ik zal zo nog even kijken naar de vertaling!