I was having the same issue and the following instructions solved my problem:
1) Editing the files:
for 64 bits:
sudo gedit /usr/lib/x86_64-linux-gnu/gtk-3.0/3.0.0/immodules.cache
sudo gedit /usr/lib/x86_64-linux-gnu/gtk-2.0/2.10.0/immodules.cache
for 32 bits:
sudo gedit /usr/lib/i386-linux-gnu/gtk-2.0/2.10.0/immodules.cache
changing the line
"cedilla" "Cedilla" "gtk20" "/usr/share/locale" "az:ca:co:fr:gv:oc:pt:sq:tr:wa"
to
"cedilla" "Cedilla" "gtk20" "/usr/share/locale" "az:ca:co:fr:gv:oc:pt:sq:tr:wa:en"
(note the "en" at the very end)
2) replacing "ć" to "ç" and "Ć" to "Ç" on /usr/share/X11/locale/en_US.UTF-8/Compose (if you use more locales, you may need to this replacement for all of them)
sudo cp /usr/share/X11/locale/en_US.UTF-8/Compose /usr/share/X11/locale/en_US.UTF-8/Compose.bak
sed 's/ć/ç/g' < /usr/share/X11/locale/en_US.UTF-8/Compose | sed 's/Ć/Ç/g' > Compose
sudo mv Compose /usr/share/X11/locale/en_US.UTF-8/Compose
3) add two lines on /etc/environment
GTK_IM_MODULE=cedilla
QT_IM_MODULE=cedilla
4)restart your computer
Source: https://bugs.launchpad.net/ubuntu/+bug/518056/comments/39