Comment # 2 on bug 1158708 from Stefan Hundhammer

I just did a new TW installation to test this, and it worked fine for me; see
also the attached screenshots.

But there is a caveat: You have to make that selection in the right places. You
need to add the new language in YaST, and then you need to tell your desktop
(KDE Plasma 5 in your case AFAICS) to use that language; because your user
account is already created, so the desktop already has settings for what
language to use. YaST only changes the SYSTEM language which is independent
from existing user accounts; it will be used for any additional user accounts
that are created later.

And then there is that old topic of recommended packages: If you leave the
defaults, you will get packages installed that have a "recommended" package
dependency; i.e. if you install package foo-office, you will also get package
foo-office-lang which will provide translations. If you turn that setting off,
you will not get those additional packages (which is the whole point of that
setting), so you will not have translations.

I am quite confused to see that you get translations for the YaST NCurses
control center (the text based version). But that might depend on subtle
differences how environment variables like $LANG and $LC_MESSAGES are used; the
YaST Qt control center is a completely separate piece of code.

Exactly what did you do?
What is the output of the "locale" command when you run it as a normal user?
What is the content of your /etc/sysconfig/language file? (please attach)