Loads the translated strings for a plugin residing in the mu-plugins directory.

function load_muplugin_textdomain( $domain, $mu_plugin_rel_path = '' ) {
	/** @var WP_Textdomain_Registry $wp_textdomain_registry */
	global $wp_textdomain_registry;

	/** This filter is documented in wp-includes/l10n.php */
	$locale = apply_filters( 'plugin_locale', determine_locale(), $domain );

	$mofile = $domain . '-' . $locale . '.mo';

	// Try to load from the languages directory first.
	if ( load_textdomain( $domain, WP_LANG_DIR . '/plugins/' . $mofile, $locale ) ) {
		return true;
	}

	$path = WPMU_PLUGIN_DIR . '/' . ltrim( $mu_plugin_rel_path, '/' );

	$wp_textdomain_registry->set_custom_path( $domain, $path );

	return load_textdomain( $domain, $path . '/' . $mofile, $locale );
}