GTK apps in wrong language after switching main language

I’ve run the installation in Polish and later switch to US English as the system-wide language with the exception of the keyboard input which I kept Polish. All KDE apps are using English now. But not the GTK apps (GIMP, Inkscape, Genius Math Tool) - they still display things in Polish. Also output in Konsole was Polish until I’ve edited the .bashrc file by adding (to have English on output, but keep the ability to write Polish):

export LANG=pl_PL.utf8
export LANGUAGE=en_US

When I run a GTK app from the terminal then it’s in English.

People used to report this problem:
but this have not helped me much. Seems to be an upstream issue in KDE or at least Kubuntu, as same things happens in Kubuntu 13.10.