Request.UserLanguages is the property you’re looking for. Just keep in mind that this array may contain arbitrary (even non-exsitent) languages as set by request headers.
UPDATE
Example:
// Get Browser languages.
var userLanguages = Request.UserLanguages;
CultureInfo ci;
if (userLanguages.Count() > 0)
{
try
{
ci = new CultureInfo(userLanguages[0]);
}
catch(CultureNotFoundException)
{
ci = CultureInfo.InvariantCulture;
}
}
else
{
ci = CultureInfo.InvariantCulture;
}
// Here CultureInfo should already be set to either user's prefereable language
// or to InvariantCulture if user transmitted invalid culture ID