Language files for Cockpit's backend
Just copy the language files you need to /config/cockpit/i18n. After that you will be able to change the language of Cockpit users in the user management.
When the folder /config/cockpit/i18n is missing, just create it. Keep in mind to keep webserver permissions for those folders.
You can create the /config/cockpit/i18n folder and a ready to translate file executing the following cli command:
./cp create-lang --lang [code]