Also I have just noticed (by chance from looking at a diff) that in functions_module.php on Line 452:
trigger_error('Module not accessible', E_USER_ERROR);
and Line 460:
trigger_error("Cannot find module $module_path/{$this->p_name}.$phpEx", E_USER_ERROR);
and Line 467:
trigger_error("Module file $module_path/{$this->p_name}.$phpEx does not contain correct class [{$this->p_name}]", E_USER_ERROR);

Also looking at http://tracker.phpbb.com/browse/PHPBB3-9975 I have a feeling that this could run deeper that a lot of errors don't use language variables.