Language Interface not consistent

In drupal 5, I set my site's language as Chinese. in my administrator account's profile, I selected English as Language.

After I upgrade to drupal 6, I realized it was no longer behaving like before.

I have done the following:

1. set Chinese as default langauge, select English as language preference in my administrator account profile.
2. select "Path prefix with language fallback. " as option.
3. set language domain of Chinese as "www.mysite.com"
4. set language domain of English as "en.mysite.com"

the results:

the site will display Chinese language interface most of the time, but it will display English interface randomly for anonymous users.

What I am looking for is the SAME behavior as drupal 5.

could anyone point me a correct direction to have this done?

Thank you!