I had the same problem. I am a Fedora newbie and used the KDE live CD (from a USB pen drive) to install Fedora 17 on a UEFI GPT system (in dual boot with Windows 7).
I did set the system language to German, but KDE was still all in English.
I used the Konsole to run the following:
# su -
# yum search german
# yum install kde-l10n-German
# yum install calligra-l10n-de
# yum install man-pages-de
You can also put this in one line: yum install kde-l10n-German calligra-l10n-de man-pages-de
Depending on what you have installed, you may also need other packages (have a look at the list you get with yum search german
I don't know if KDE would do this automatically, but from System Settings – Locale – Country/Region & Lanugage
I added "Deutsch" to Preferred Languages. That did the trick (after a restart).