Installed plugin but still English

Do you have an Dutch WordPress installation?
https://nl.www.ads-software.com/

Check your wp-config.php file on:

define ('WPLANG', 'nl_NL');

No, I have an English WP installation (as I described in my question), would like to keep it that way, only need NL translation for the Gravity Forms that I use.

The “Gravity Forms (nl)” plugin only works for a Dutch WordPress installation.

Mmm … ok … welll, just a little hint: please state that somewhere in the main product description ??

Thanks, will be added in the next release:
https://github.com/pronamic/wp-gravityforms-nl/commit/92785008159345bb90e14a393d4ccb93f8f21968