thank you very much for your suggestion! The Genesis Translations plugin does the trick. It reads from wp-config.php which language I run WP in (de_DE) and uses the matching translation file. Nothing to set up, it just works right out of the box :-).
I thought I needed a translation for the child theme. Google sent me to David Decker`s site first but there was no translation for Optimal.
Again many thanks for your help!