_get_path_to_translation_from_lang_dir

 * to call load_plugin_texdomain() or load_theme_texdomain().
 *
 * @since 4.6.0
 * @access private
 *
 * @see get_translations_for_domain()
 * @global MO[] $l10n_unloaded An array of all text domains that have been unloaded again.
 *
 * @param string $domain Text domain. Unique identifier for retrieving translated strings.
 * @return bool True when the textdomain is successfully loaded, false otherwise.
 */
function _load_textdomain_just_in_time( $domain ) {
	global $l10n_unloaded;

	$l10n_unloaded = (array) $l10n_unloaded;

	// Short-circuit if domain is 'default' which is reserved for core.
	if ( 'default' === $domain || isset( $l10n_unloaded[ $domain ] ) ) {
		return false;
	}

	$translation_path = _get_path_to_translation( $domain );
	if ( false === $translation_path ) {
		return false;
	}

	return load_textdomain( $domain, $translation_path );
}

/**
 * Gets the path to a translation file for loading a textdomain just in time.
 *
 * Caches the retrieved results internally.
 *