Download the WordPress language file. The Dutch files can be found on the Dutch WordPress site
Extract the tar-file and move al the languages files (*.mo, *.po) to their corresponding locations of the WordPress site (/wp-content/languages/ etc).
Install the plugin WP Native Dashboard. Now you can choose which language you want to use in the backend.
Pitfalls
I experienced problems with the content of pages when the option Translate Frontend Admin Bar using backend selected language was selected (infinite scrolling, no admin bar).