Change logged-in statusmessage translations and add German translations

hey there,

- I have changed the `Welcome. You are now logged in.` message to use the `plone` domain rather than the one of this package. This has the advantage that it is already translated into most languages.

- I've also updated the `.pot`-files and added German translations. The other translations seem a little out of date - there are quite a lot of changes in the `.pot`-file.

Thanks for this packages, by the way. It works no very well in our site :wink: