As I understand it, you have to do something like this:

$lang_files = $finder
                ->prefix('permissions_')
                ->suffix(".$phpEx")
                ->extension_directory('/language/' . $user->lang_name)
                ->core_path('language/' . $user->lang_name . '/mods')
                ->find();

If we set the core_path as /language/ folder, it will search all files within the core language directory but I don't know if that's acceptable or not, otherwise, we need to set the core_path to /mods/, then /acp/