I have a few international sites running umbraco where I've created a class inheriting from IHttpModule and in the contextBeginRequest() method setting the following:

string Lang = "fr-FR";

Thread.CurrentThread.CurrentCulture = CultureInfo.CreateSpecificCulture(Lang);
Thread.CurrentThread.CurrentUICulture = new CultureInfo(Lang);

I can see the breakpoints hit on these lines when debugging, but they no longer control the dictionary.  Interestly, I found a way around this issue by adding the following Razor code to my site template:

<umbraco:Macro runat="server" language="cshtml">
    @Helper.SetLanguage()

</umbraco:Macro>

SetLanguage() executes:

string Lang = "fr-FR";
Thread.CurrentThread.CurrentCulture = CultureInfo.CreateSpecificCulture(lang);        Thread.CurrentThread.CurrentUICulture = new CultureInfo(lang);

Thanks,

-Carter