Comment # 3 on bug 1208249 from Shawn Dunn

You need to install the fr_FR.UTF8 locale.  (Or whichever localization of
french that it is you use.)

sudo transactional-update run zypper addlocale fr_FR.UTF8

Reboot, and then set your language preferences in the kcm, and logout and back
in, and Plasma will be in French.