actually this is what i thought as well, but after looking at its implementation. I dont think it will ever return true. See following.
///phpbb3/phpBB/includes/functions_user.php:
/**
* Validate Language Pack ISO Name
*
* Tests whether a language name is valid and installed
*
* @param string $lang_iso	The language string to test
*
* @return bool|string		Either false if validation succeeded or
*							a string which will be used as the error message
*							(with the variable name appended)
*/
function validate_language_iso_name($lang_iso)