I found some reference that you should install the package qt4-ibus. Do that, then open your file manager and press Ctrl+H to show hidden files also. Edit the now displayed .profile file in your home folder with Kate, and at the end append this line:
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export QT_IM_MODULE=ibus
Save and close the file. Then from the menu open Autostart, and add the program IBus to be automatically started (you will find IBus in the System section). Then log out and log in again and Ctrl+Space should work if you have a program open that supports text entry. You should also have a new icon on your panel, which you can right-click to change which keyboard entry method is active.
I managed to type some Bengali, though I couldn't read what I was typing

Should IBus still not work, another suggestion was to install the package qt4-qtconfig, and then run qtconfig and configure on the Interface tab the Default Input Method to ibus. It may not have been set correctly, though for me it was and I didn't need to do this.