A server that receives a request for a document with a preferred language of 'en-GB, fr' when both an 'en' and 'fr' version exist will serve the French one. It will only serve the English document before the French one if there is a version of the file with en-gb for the language extension. It does work the other way though, e.g. a server can return en-us when en is requested. We strongly recommend that you do not add country extensions to a language unless you have good reason . If you do add one, make sure you also include the language without the extension.
Many people route web requests through a cache.
This is essentially a web server that has no content of its own. It sits in the middle between users and real web servers. The cache grabs requests for web pages, fetches the page and makes a local copy. This can really cut down on net traffic when many users request the same page. This is a great idea most of the time, but fails when the cache is buggy.