Automatic language-culture selection and fallback algorithm in ASP.NET - asp.net

Question

We are building a site that supports several languages. We have created resources to support fr, de, and en-US with en-US as the default. I have added:
<system.web>
<globalization
uiCulture="auto" culture="auto"
requestEncoding="utf-8"
responseEncoding="utf-8"
enableClientBasedCulture="true" />
...
</system.web>
to my web.config file. If I set the language to fr, de, or en-US in IE or Chrome, I get the expected language. If I set the language to ar-EG, I get en-US, also as expected.
If I set my language to fr-FR, but do not include fr in the list of accepted languages, the site returns the fr page, even if I tell the browser that I prefer accept de to generalized fr (as opposed to fr-FR). This appears to be contrary to the RFC's, but is not surprising, because IE defaults to fr-FR in France and de-DE in Germany, and does not work with sites that do not automatically generalize the culture.
However, if I tell the browser that I accept ar-EG and fr, the site defaults to en-US -- even though the Accept-Language header specifies that I will accept fr with a higher priority. I captured the request header in the browser to make sure all the languages I asked for were being sent with the right priorities.
It appears that ASP.NET is only ever looking at the first language specified in the Accept-Language header. Is the ASP.NET language matching algorithm specified anywhere? Is there a way to specify that ASP.NET/MVC should check all the languages in the Accept-Language list to determine the best one? Or do I have to write my own language matching code?

Answer 1

Well, as nobody else answered yet, I make my suggestion. There is a better or at least more beautiful solution for sure, but for my needs it was enough. Just set the culture inside your Application_AcquireRequestState() event:
protected void Application_AcquireRequestState()
{
var language = "whatever"; // default fall back
if (request.UserLanguages.Length > 0)
{
var acceptedLangs = new List<string> {"de", "en"};
var langs = request.UserLanguages.Where(l => acceptedLangs.Any(al => al.Equals(l.Substring(0, 2))));
language = langs.FirstOrDefault();
}
// TODO: may be better inside a try..catch block
var culture = new CultureInfo(language);
Thread.CurrentThread.CurrentUICulture = culture;
Thread.CurrentThread.CurrentCulture = culture;
}