We need to check some language related matters

[koenedaele]

Atramhasis views get the current user's language from the request.locale_name parameter. This gets the locale_name from a LOCALE GET parameter or a cookie and falls back to the default locale.

Pyramid_skosprovider, which powers most of the JSON services, apart from the tree, also uses the request.locale_name, but allows it to be overridden with the language GET parameter.

Further complication: request.locale_name might look like en_US, while language should be IANA en-US. Unless the language tags lib handles the underscores.

Other complication: request.locale_name has a function to set a cookie that only allows setting it to the configured languages, which are the UI languages. Even though the provider might support other languages.

Other consideration, do we differentiatie between the request.locale_name that tells us what language the user's browser is configured for (applying to the interface) and a parameter like language that might tell us the user wants Latin for the concept labels, even though the UI should be in English? (Possibly related to #379 )