I have set the following environment variables in my
~/.bashrc and ~/.xsession:

export LANG=zh_CN.UTF-8
export LC_ALL=zh_CN.UTF-8

same environment varialbes are set in ~root/.bashrc and ~root/.xsession,
and the contents of /etc/environment is:

$ cat /etc/environment
LANGUAGE="zh_CN:zh:en_US:en"
LANG=zh_CN.UTF-8

Every thing works fine when I edit utf-8 encoded files with vim or nano,
either as a normal user or as root.

But when I do "sudo vi filename" as a normal user, the chinese characters
that I input is completely messed up, I must use the "set encoding=utf8"
command to make vim work normally.

And problem is even worse with "sudo nano filename", because I have no way
to make nano work normally with chinese character input.