I try to override the standard email validation message: ‘Please enter a valid email address.’ with a message like: Email is een verplicht veld.

SO I try it like this:

add_action('bp_core_validate_email_address', function (WP_Error $errors, $validation_results) {
    $bp = buddypress();   
  

    if (isset(( $validation_results['signup_email']))) {
        $errors->add['signup_email'] = sprintf(
            '<div class="bp-messages bp-feedback error">
								%s

							</div>',
            'Email is een verplicht veld.'
        );
    }
});