E-Mail template override or filters

Hi there, I already posted on the WP forum, but haven't received an answer yet.
I am trying to modify the emails that are sent through your plugin. It seems that there is no possibility of a template override (which would be awesome btw). I found the filter "user_verification_email_templates_data", but I seem not to be able to make it work for me. I am trying this:

add_filter( 'user_verification_email_templates_data', 'custom_user_registered' );
function custom_user_registered( $templates_data ) {
$templates_data['email_resend_key']['subject'] = "Test";
return $templates_data;
}

But when I resend the mail the subject of the mail doesn't change to "Test". What am I missing or doing it wrong? Maybe you could point me in the right direction?

Thank you so much in advance!
Ben