Are there localization files available for the plugins? Especially notification plugin with its systray icon menu and so on
Yes, there are. At least, for those plugins which have the infrastructure for it. But not all of these are translated into German yet.
> But not all of these are translated into German yet. How can I help? As said, I would like to see the notification plugin in german.
Please see this page: http://www.claws-mail.org/i18n.php join the translators' ml, read the Translator's Information page, and contact the current German translator. Thanks!