Ok, fixed it by deleting everything from the server after backing up wp-config, the theme, uploaded data and some other stuff (analytics and such). Then copied the Dutch version to the server, updated the wp-config (with define('WPLANG', 'nl_NL');
) and uploaded that plus all the other stuff as well.
That fixed it, but shouldn't be the way to do things according to the WP site.
Recommendation to others wishing to switch langauge is to take this into consideration, for if the event might occur. Also, don't make any edits to the DB during this operation and it should continue to function as if nothings changed.