REST API should obtain language settings from Client

(Noted by Andrea Bollini (4Science) in today's DSpace 7 Mtg)

While users can change language preference/settings in the Angular UI layer for DSpace 7, the backend (REST API) is never notified of these language preferences.  While most i18n (internationalization) settings exist in the UI layer, there are some backend features which also require i18n, namely:

• Submission forms
• Controlled Vocabularies
• Authorities

Ideally, when a user changes languages in the UI layer, that information should be passed via a request to the REST API.  To remain stateless, this setting may need to be passed on every request or perhaps store it in a Cookie.

The early recommendation is to perhaps support it via the "Accept-Language" Header: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept-Language  However, we need to determine if this is feasible for clients like the Angular UI, and whether it will meet our needs.