Add discriminant language or country to the data model

Starting point, this thread : https://github.com/dis-moi/backend/issues/343#issuecomment-819313978

To do before : https://github.com/dis-moi/backend/issues/343 (same issue)

Start by reading issue 343 and the linked comment, then inspect the existing data model and its API representation. Clarify whether the discriminant is language, country, or both, where it belongs, and which tests or consumers must change; done means the agreed model is implemented and its behavior is covered.

Written by the indexing model from the issue text.