Fixes for the Dutch translation files

I noticed some translations for Dutch were missing when working on my wife's book (https://leanpub.com/twinspiratie). I found this repository, and figured I'd help out. Let me know if this is the wrong place to do that!

Translations in the Dutch files seemed mostly google-translated. I've fixed them, updated word-use (there were some archaic constructs in there), made use of 'u'/'je' consistent (you/thou, everything is 'u' and 'uw' now), and made things into readable sentences...