> but only for en-GB not taking into account the default admin language
That's because the API is a separate application. Just like you have the installation application, the admin application, the frontend application, the API has been implemented as a separate Joomla application.
And so it's not aware of the Joomla frontend or backend, and their language.
When you use the API, I think it helps understanding how it works if you consider that you are using from an app on a mobile phone: it does not know anything about the site context.
That's also why if you logged-in to the admin or frontend, and building a component with the API, your API calls are not going to be "logged-in". Sessions are not shared between the API application and the others.