I have put nl_NL as the language in the configuration file, and most of the site is now indeed in Dutch.
I also expected plugins wchich have translation files would pick this up. I am using Contact Form 7 which has the file wpcf7-nl_NL.mo in its languages directory. But it is not being picked up.
Als the theme I use has a nl_NL.mo file, but again, it does not show up in dutch at the website.
Have I forgotten something or is this a bug?