Translate the GeneratePress 404 page title using Polylang strings translation

generatepress_polylang_404.php

/**
 * Translate the GeneratePress 404 page title
 *
 * The string can be translated in Languages -> Strings translations
 */
if ( function_exists( 'pll_register_string' ) ) {
	function mattrad_404_text() {
		$error_text = 'That page cannot be found';
		pll_register_string( '404', $error_text , 'text_domain');
		return pll__( $error_text );
	}
	add_filter( 'generate_404_title', 'mattrad_404_text' );
}