All feedback from the TrustedSelfCheckout is untranslatable, becuase of single underscores vs. double underscores in a .js file: https://git.koha-community.org/Koha-community/Koha/src/commit/26203a66098fdd610e6a1a5b1782e9f46cd33d07/koha-tmpl/opac-tmpl/bootstrap/js/modals/checkout.js#L43
System preference is name: OpacTrustedCheckout