Why are any Chinese characters replaced with question marks in vsftpd.log?

[Fluid]

Why are any Chinese characters replaced with question marks in vsftpd.log?
 (some directory names and file names have multibyte chars)

Server:
FreeBSD 7.0 Release minimal installed only with bin and doc
installed vsftpd 2.0.5 using Release package

/etc/rc.conf
defaultrouter=XXX
hostname=XXX
ifconfig_em0=XXX
inetd_enable="NO"
sshd_enable="YES"

Client:
Windows XP sp2 Simply Chinese Edintion
FTP client : Internet Explorer OR CuteFTP 8 Professional

How to repeat this problem:

1. Client upload/download files(Chinese Name) to the Server
2. view /var/log/vsftpd.log in FreeBSD or Windows.
The Chinese file names in vsftpd.log are replaced with question marks.

If using ftpd or lukemftpd
The Chinese file names in vsftpd.log can be viewed correctly.

I've searched google but only get one result leaving this problem unsolved
http://ubuntuforums.org/showthread.php?t=346756

--question---
How to make vsftpd.log logging Chinese file names correctly?