SCIM stands for 'Smart common input method' (http://www.scim-im.org). The SCIM engine can act as an input method for any language, including complex languages such as Chinese. It is easy to use SCIM tables to create a custom keyboard.
SCIM is a useful alternative to xkb for switching between keyboard layouts. SCIM provides some functions that xkb does not. I personally use it to type ancient Greek, pointed Hebrew, and Chinese.
SCIM is no more difficult to setup in XFCE than in Gnome or KDE. The easiest, user-level way to set up SCIM is to edit the .Xsession file:
LANG=en_US.UTF-8 export LANG XMODIFIERS="@im=SCIM" export XMODIFIERS GTK_IM_MODULE="scim" export GTK_IM_MODULE scim -d& startxfce4
Set LANG to your own local language, of course. SCIM should work as long as the locale is UTF-8. After doing this, SCIM will work in XFCE by default.
Good luck, and have fun.