Hi Jörg,
To add search button to your language “suche” you need to upload translation file under language folder. Before uploading first of all create a child theme and then only add your .po and .mo file inside the language directory.
If you like to change it directly without creating .po and .mo file then the file is inside theme directory-> inc-> structure-> searchform-extension.php and translate to your language. Before making any change first make child theme to be safe from data loss while updating to our new version. If you could not make the child theme then each and every time while updating to our new version you need to edit the same thing.
Thank you!