Hi, I switched to Kinoite a few days ago, ran into the same issue and tried to figure out what was going on. I noticed this:
~ $ rpm -q --list plasma-discover | grep locale/de
/usr/share/locale/de/LC_MESSAGES/kcm_updates.mo
/usr/share/locale/de/LC_MESSAGES/plasma-discover.mo
~ $ ll /usr/share/locale/de/LC_MESSAGES/plasma-discover.mo
/usr/bin/ls: cannot access '/usr/share/locale/de/LC_MESSAGES/plasma-discover.mo': No such file or directory
~ [2] $ ll /usr/share/locale/de/LC_MESSAGES/kcm_updates.mo
/usr/bin/ls: cannot access '/usr/share/locale/de/LC_MESSAGES/kcm_updates.mo': No such file or directory
~ [2] $ ll /usr/share/locale/en_GB/LC_MESSAGES/plasma-discover.mo
-rw-r--r--. 4 root root 11K Jan 1 1970 /usr/share/locale/en_GB/LC_MESSAGES/plasma-discover.mo
To me it doesnt look like one individual locale package is missing, but instead rpm decided to filter out the language files for everything != english out at installation time. I found the rpm %_install_langs macro and the install-langs option in the rpm-ostree docs which appear to do exactly that. Maybe one of these leaked into the Kinoite build?