I have added the following to web.config to override the thread culture of the site but it does not take effect, the browser setting is still used:

<add key="UseCultureOverride" value="true" />

<!-- First site shall always be in English -->
<add key="site1culture" value="en-US" />

<!-- Site ID=6 shall be zh-CHT -->
<add key="site6culture" value="zh-CHT" />

<!-- Site ID=7 shall be zh-CHS -->
<add key="site7culture" value="zh-CHS" />

There are no overriding settings in user.config for these keys. Note: I am using <add key="UseRelatedSiteMode" value="true" />, if it helps.

Any directions on how to diagnose?

Thanks Joe, good point and I moved the settings to user.config instead.

Unfortunately the different sites still use the browser setting. It just seems like culture override is not enabled. I even restarted the web server. So all sites are using resource files which are English.

Is there any other setting that may prevent culture override from working?

I know, I am sure that other settings take effect in both web.config and user.config but not so for culture.

Actually, after some research I figured out the problem. It was more subtle than which config file, etc. There is a CultureHelperHttpModule.cs implementation that reads the culture setting. It is very straightforward. However you catch all exceptions without logging when overrideing CurrentThread so we do not know if anything special happened. I have the module definition set like this, like the default:       <add name="CultureHelperHttpModule" type="mojoPortal.Web.CultureHelperHttpModule, mojoPortal.Web" /> so surely this module runs on page load.

When I was investigating this I noticed something odd in the Performance viewer on the server. Every time I request or refresh a web page from a site which is forced to non-English the performance counter "# of Exceptions" increases by one. However, as soon as I set UseCultureOverride to false, there is no such exception registered in the performance counter!

Just for fun, I then set all culture overrides to ja-JP. Then, there were no exceptions, and all resources were displayed correctly in Japanese!

It seems that the problem arised since I used zh-CHS and zh-CHT. Instead of zh-CHT I switched to use zh-HK, which worked properly for Traditional Chinese override. I also changed to zh-CN instead of zh-CHS which worked properly for Simplified Chinese.

There is an interesting note on MSDN saying: "The CultureInfo..::.CreateSpecificCulture method throws ArgumentException for the neutral cultures "zh-Hant" ("zh-CHT") and "zh-Hans" ("zh-CHS"). " Perhaps this happens in the application now, but the exception is swallowed. See:
http://msdn.microsoft.com/en-us/library/system.threading.thread.currentuiculture.aspx 

You can check in your own test, if you force the site culture to zh-CHT and zh-CHS, I would be surprised.

Since the resource files are called zh-CHT and zh-CHS it might be worth pointing out that these cannot be used as override cultures for a site.