Hallo Ingo,
Thanks for reaching out to us!
The version you are using is likely an older version, which in not yet translatable through a .po file unfortunately.
The only thing you can do is edit the code manually and change the texts to German.
Also, to make it open in a new window, you will likely need to edit this line in the code:
%2$s
and change it like this:
%2$s
Please note, I haven’t tested this and I don’t know what version you exactly have, so this is more a guideline.
Also note, there is already a newer version of the plugin. You can find it here if you want to try it. I suggest though to make a copy of your current one, in case that works out better for you. The translation in the new one is also not possible yet, but it is on our roadmap to make them translatable.
Cheers,
Andras