As a Developer, I can specify which language I want.
I can include the locale in the REST request in some way to specify which language response I want.
I can request to get the response in all locales, by not requesting a specific language.
Currently, every response includes the content from all locales.
Christoph Meier was collecting ideas for localization of the JSON provided by the delivery endpoint in the context of the ideation (of PM); see Prototyping: Hybrid Headless - cmeier - the linked google doc also provides linked to earlier research done by the VN core team.