Unlike extra plug-ins and dnf itself, translations for dnf-plugins-core have not been pulled from/pushed to https://fedora.zanata.org/project/view/dnf-plugins-core in a year.
https://github.com/rpm-software-management/dnf-plugins-core/pull/205
dnf-plugins-core-1.1.0-1.fc26 has been submitted as an update to Fedora 26. https://bodhi.fedoraproject.org/updates/FEDORA-2017-f56311f5dd
dnf-plugins-core-1.1.0-1.fc26 has been pushed to the Fedora 26 testing repository. If problems still persist, please make note of it in this bug report. See https://fedoraproject.org/wiki/QA:Updates_Testing for instructions on how to install test updates. You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2017-f56311f5dd
dnf-plugins-core-1.1.0-1.fc26 has been pushed to the Fedora 26 stable repository. If problems still persist, please make note of it in this bug report.