https://www.w3.org/International/questions/qa-lang-priorities talks about the negotiation between browser and server:

The basic rule here is that if your language preference list contains a language tag containing a hyphen, such as fr-CH (French as spoken in Switzerland), you should consider adding an additional language tag without the hyphen, ie. fr (French) in this case, immediately after.

This is because if a server follows the HTTP/1.1 specification literally, it is unable to match fr-CH against a document labelled on the server as fr.

In our case the server side is awaiting for a second (separate) parameter country however to allow direct interaction we should allow to detect region/country locale preferences from the language parameter.