Hey Adam,
now it works! Thank you so much for helping me! ??
// Neue WPJM Job Types in Deutsch
add_filter( 'wpjm_job_listing_employment_type_options', 'akh_add_new_wpjm_employment_type' );
function akh_add_new_wpjm_employment_type( $employment_types ) {
$employment_types['Vollzeit'] = __( 'Vollzeit', 'wp-job-manager' );
$employment_types['Teilzeit'] = __( 'Teilzeit', 'wp-job-manager' );
$employment_types['Aushilfe'] = __( 'Aushilfe', 'wp-job-manager' );
$employment_types['Befristet'] = __( 'Befristet', 'wp-job-manager' );
$employment_types['Praktikum'] = __( 'Prakitkum', 'wp-job-manager' );
$employment_types['Ausbildung'] = __( 'Ausbildung', 'wp-job-manager' );
$employment_types['Studium'] = __( 'Studium', 'wp-job-manager' );
$employment_types['Messe'] = __( 'Messe', 'wp-job-manager' );
unset( $employment_types['FULL_TIME'] );
return $employment_types;
}