Comment # 8 on bug 932158 from Wolfgang Bauer

Well, I noticed today that there's an additional problem, not only the missing
patch mentioned already (to read from desktop-translations):
The Plasma5 packages install the translations to /usr/share/locale/kf5/, where
they apparently are not found.
The package ki18n does contain a patch to look for translations in
/usr/share/locale/kf5/ (and
"/usr/share/locale-bundle/kf5"/"/usr/share/locale-langpack") too. But that
either doesn't work (any more), or is not sufficient for all situations where
translations are loaded.

Copying the mo files from /usr/share/locale/kf5/XX/LC_MESSAGES/ to
/usr/share/locale/XX/LC_MESSAGES/ should fix most translations (except those
that are in a *.desktop file directly).