If you expect to type in Portuguese using the "English (US, alternative international)" keyboard layout because you have a US or ANSI keyboard instead of ABNT2 that is the standard keyboard adopted here in Brazil (a keyboard profile that I personally hate), here is the solution (that I never understood why it was not corrected on any Debian-based Linux yet):
 

Open Terminal as root or "sudo su" then according to your version of Linux Mint...

(If 64 bits...)

sed -i "s/az:ca:co:fr:gv:oc:pt:sq:tr:wa/az:ca:co:fr:gv:oc:pt:sq:tr:wa:en/" /usr/lib/x86_64-linux-gnu/gtk-3.0/3.0.0/immodules.cache
sed -i "s/az:ca:co:fr:gv:oc:pt:sq:tr:wa/az:ca:co:fr:gv:oc:pt:sq:tr:wa:en/" /usr/lib/x86_64-linux-gnu/gtk-2.0/2.10.0/immodules.cache
sed -i "s/ć/ç/g" /usr/share/X11/locale/en_US.UTF-8/Compose
sed -i "s/Ć/Ç/g" /usr/share/X11/locale/en_US.UTF-8/Compose
    # echo "GTK_IM_MODULE=cedilla" >> /etc/environment
    # echo "QT_IM_MODULE=cedilla" >> /etc/environment

(If 32 bits...)

sed -i "s/az:ca:co:fr:gv:oc:pt:sq:tr:wa/az:ca:co:fr:gv:oc:pt:sq:tr:wa:en/" /usr/lib/i386-linux-gnu/gtk-3.0/3.0.0/immodules.cache
sed -i "s/az:ca:co:fr:gv:oc:pt:sq:tr:wa/az:ca:co:fr:gv:oc:pt:sq:tr:wa:en/" /usr/lib/i386-linux-gnu/gtk-2.0/2.10.0/immodules.cache
sed -i "s/ć/ç/g" /usr/share/X11/locale/en_US.UTF-8/Compose
sed -i "s/Ć/Ç/g" /usr/share/X11/locale/en_US.UTF-8/Compose
    # echo "GTK_IM_MODULE=cedilla" >> /etc/environment
    # echo "QT_IM_MODULE=cedilla" >> /etc/environment