Extra dutch translation string

interface/web/login/lib/lang/nl.lng

@@ -16,7 +16,7 @@ $wb['username_txt'] = 'Gebruikersnaam';
 $wb['password_txt'] = 'Wachtwoord';
 $wb['login_button_txt'] = 'Inloggen';
 $wb['pw_lost_txt'] = 'Wachtwoord vergeten';
-$wb['error_maintenance_mode'] = 'This ISPConfig installation is currently under maintenance. We should be back shortly. Thank you for your patience.';
+$wb['error_maintenance_mode'] = 'Deze dienst is momenteel in onderhoud. We zijn spoedig weer beschikbaar.';
 $wb['login_txt'] = 'Inloggen';
 $wb['pw_reset_txt'] = 'Wachtwoord herstellen';
 $wb['pw_button_txt'] = 'Wachtwoord versturen';