Custom attrs hreflang "x-default" Polylang

custom_attrs_polylang.php

/* ---------------------------------------------------------------------------
 * Set hreflang="x-default" according to Google content guidelines with Polylang
 * --------------------------------------------------------------------------- */
function filter_pll_rel_hreflang_attributes( $hreflangs ) {
	foreach ( $hreflangs as $lang => $url ) {
		if ( $lang === 'en' ) {
			printf( '<link rel="alternate" href="%s" hreflang="%s" />' . "\n", esc_url( $url ), esc_attr( 'x-default' ) );
		}
	}
    return $hreflangs; 
}
add_filter( 'pll_rel_hreflang_attributes', 'filter_pll_rel_hreflang_attributes', 10, 1 );