API responses generally contain content in one language/translation. The API user can request a language by passing an Accept-Language header.
In special cases where the frontend can't really pass HTTP headers to the API, like when a HTML response should be displayed in an iframe, then the endpoint may also take a
lang query parameter with the same syntax as the
Accept-Language header. See the respective documentation if this is possible.
In case the API returns an error it is recommended to use the contained
relay:errorId and map it to a custom error message (+ translation) provided by the frontend, instead of displaying the contained error message as is.