As stated; remembered the same thing from way back.
Had it sorta working with deadkeys and
GTK_IM_MODULE=xim in /etc/environment.
Discovered that otherwise it would only work in some applications like Firefox, or so it seemed.
xim is old though and has other drawbacks from what I've read.
Did minimal install myself so no IBus whatsoever on my PC.
sudo apt-get install ibus
Added ibus-daemon to startup applications through menu
added the following lines to /etc/environment below path line:
sudo nano /etc/environment
(as stated by ibus-setup output)
And got it working too
You probably had IBus already running than, but other input method? Or none?
Anyway; glad to help out, enjoy the ć's and ç's!