What is more, here is my set command output:
--snip--
GTK_IM_MODULE=iiim
--snip--
LANG=pl_PL.UTF-8
--snip--
QT_IM_MODULE=iiimqcf
--snip--
XMODIFIERS=@im=iiimx
So all necessary variables are set. But for some reason ctrl-space does not work
I have switched to scim and now everything works flawlessly!