Add locale in api reponses

[mattiaz9]

Right now the elements returned from an api call are automatically translated depending on the query param ?locale=.
The problem is when a document doesn't have a translation and the fallback locale is used, so there is no way to know what locale is used.

That is important when composing a url, not knowing the correct locale could result in a 404.

Can you please add it in the api response of each element?