Comment # 2 on bug 1095208 from Wolfgang Bauer

(In reply to Wolfgang Bauer from comment #1)
> So, is systemsettings5-lang or a bundle-lang-kde-xx package installed?

I just saw that the bundle-lang packages have finally been dropped from
Tumbleweed as well (they are not used in Leap anymore since years).

That probably caused your problem.

So, install systemsettings5-lang and you should be able to select other
languages in systemsettings5.

Or even better, run "zypper inr" to install all recommended packages, that
would also include all translations (which are now only available in xxx-lang
packages).