I had the same problem after a fresh Kubuntu 13.04 installation. The installer sets the locales based on the geographical location supplied by the user.
You can see the current locales with:
It seems that Qt checks the LC_NUMERIC variable to set the language, but it also works with LC_ALL, change both in /etc/default/locale to the desired local, for example "en_US.UTF-8":
After saving the changes, run:
to update the locales. It is necessary to log-out and log-in again.
More information: https://help.ubuntu.com/community/Locale