Go to your theme options page and choose the Style tab.
Scroll down to the Custom CSS box and add this:
Update your settings.
That should remove the author for the line there, but leave category and date behind.
There isn’t a bundled German translation of the theme available. You’re welcome to make one if you’d like to. There is a plugin called Loco Translate (https://wordpress.org/plugins/loco-translate/) that if it works as advertised, will allow you to make translations from inside your dashboard.