Set hreflang="x-default" for WP Globus WordPress Plugin

functions.php

<?php

/* ---------------------------------------------------------------------------
 * Set hreflang="x-default" with WP Globus
 * --------------------------------------------------------------------------- */
add_filter( 'wpglobus_hreflang_tag', 'custom_head_hreflang_xdefault', 10, 2 );
function custom_head_hreflang_xdefault() {

    $hreflangs = WPGlobus_Utils::hreflangs();
    $default_language = WPGlobus::Config()->default_language;

    $hreflangs[ $default_language ] = str_replace( $default_language, "x-default", $hreflangs[ $default_language ] );

    return $hreflangs;

}

@Jehu
With this code you get incorrect hreflang for default language, like
hreflang="x-default-US"

/** Set hreflang="x-default" with WPGlobus. */
add_filter( 'wpglobus_hreflang_tag', 'filter__wpglobus_hreflang_tag', 10 );
function filter__wpglobus_hreflang_tag($hreflangs) {
$default_language = WPGlobus::Config()->default_language;
$hreflangs[ $default_language ] = preg_replace( '/hreflang="'.$default_language.'(-|_).{2}/', 'hreflang="x-default', $hreflangs[ $default_language ] );
return $hreflangs;
}

Thank you @alexgff