FYI:

You can use Mule-UCS package for both XEmacs with MULE and FSF
Emacs 20.7/21.  It is available from:

:::The primary distribution:::
  ftp://ftp.m17n.org/pub/mule/Mule-UCS/Mule-UCS-0.84.tar.gz

:::Unofficial XEmacs package (made by me):::
  ftp://ftp.jpl.org/xemacs/packages/Mule-UCS-0.84-pkg.tar.gz

I am using the following codes in my XEmacs startup file.

(require 'un-define)
(coding-system-put 'utf-8 'category 'utf-8)
(set-coding-category-system 'utf-8 'utf-8)
(set-coding-priority-list (cons 'utf-8
				(delq 'utf-8 (coding-priority-list))))
-- 
Katsumi Yamaoka <yamaoka@jpl.org>