[Plugin: Contact Form 7] Language won't set

I have the strangest thing going on. My WordPress installation is done through the hosting providers control panel called Direct Admin.

I have chosen the dutch language.

I installed the plugin. But it won’t show the dutch language. It stays in english. I looked at the wp-config file to see if WP is really set to dutch and I see
define(‘WPLANG’, ‘nl’);
NOT
define(‘WPLANG’, ‘nl_NL’);

when I change it to nl_NL my WP admin panel changes it’s language from dutch to english and contact form 7 still isn’t in dutch.

When I change wpcf7-nl_NL.mo to wpcf7-nl.mo Contact form 7 is in dutch but when someone does not fill in a required field the error message is in english even though it has been translated in the po file (I have checked).

I tried an alternative route by translating the english text in the functions.php of contact form 7 but that also doesn’t work.

I am lost and need help.