First, you can use C, en_US and still input chinese in firefox or Gnome application. But you may not be able to type chinese in command line and some applications may be wired while handling chinese. en_HK / en_TW is better if you want to keep english interface but able to type chinese. The correct way is login the Gnome with the locale you want (choose Language in the GDM) and type "im-switch -s scim" on the command line (no need to login root) then it make in symlink on ~/.xinput.d/en_HK to /etc/X11/xinit/xinput.d/scim and the scim will be there next time you login with the same locale.

The manual way is setup the XMODIFIERS, GTK_IM_MODULE and QT_IM_MODULE env. var. before you run the applications:

export XMODIFIERS="@im=scim"
export GTK_IM_MODULE=scim
export QT_IM_MODULE=scim

and then start scim:

scim --daemon