Fixed Dutch translations

I noticed some missing translation on the site, and found this github repo. 
The Dutch translations in here seem to be Google translate generated, so I've
tried to fix them up. Made use of 'u'/'je' consistent. Made use of 
kredietkaart/creditcard consistent. I've leaned towards using 
modern/borrowed-english versions of words, as some of the used 'real' Dutch 
words seem archaic.