bookstack/resources/lang/en/errors.php

<?php

return [

    /**
     * Error text strings.
     */

    // Permissions
    'permission' => 'You do not have permission to access the requested page.',
    'permissionJson' => 'You do not have permission to perform the requested action.',

    // Error pages
    'page_not_found' => 'Page Not Found',
    'sorry_page_not_found' => 'Sorry, The page you were looking for could not be found.',
    'return_home' => 'Return to home',
    'error_occurred' => 'An Error Occurred',
    'app_down' => ':appName is down right now',
    'back_soon' => 'It will be back up soon.',
];