Hey Uwe,

Have you taken a look at this article about localisation?

You can get the translations by click the ‘Your Language‘ link on the right. Remember that the theme generates most of the text, so you will need the translation files for not only core, but theme and plugins as well.

Hope this helps, shout if you need any help..

please see our codex:  https://simple-press.com/documentation/codex/installation/installation-information/localization/