I've just switched to Mint 13 and was confronted with the problem of getting "ç". I found the answer on the internet and wanted to share it with you.
In order to fix this, a user must:
1.Edit the file: /usr/share/X11/locale/en_US.UTF-8/Compose
2.Replace the occurences of the ć with ç, for both lower and uppercase.
3.Edit the file: /usr/lib/x86_64-linux-gnu/gtk-2.0/2.10.0/gtk.immodules
4.Find the line configuring "xim", and add the "en_US" and "en" locales there, like this:
"xim" "X Input Method" "gtk20" "/usr/share/locale" "ko:ja:th:zh:en_US:en"
Of course you may have to replace "en_US:en" by something else depending on your keyboard. I replaced it with "de_DE:de" since I have a German keyboard. Then just restart X.