Add the following lines in /etc/environment
:
GTK_IM_MODULE=cedilla
QT_IM_MODULE=cedilla
Reference: https://askubuntu.com/a/370040/1244744
Create a file named .XCompose
Put in the beginning of the file include "%L" this will import the original configuration of /usr/share/X11/locale/en_US.UTF-8/Compose
Then add those lines:
include "%L"
<dead_acute><C> : "Ç"
<dead_acute><c> : "ç"
Save your file in ~/
directory
Reboot
Reference: https://askubuntu.com/a/1227109