Ok, long story short - when invoking projections e.g. 

POST against https://<SERVER>/main/ifsapplications/projection/v1/CustomerHandling.svc/CustomerInfoSet we are able to set header: Accept-Language = sv-SE but error messages are kinda mixed:
 

{

    "error": {

        "code": "ODATA_PROVIDER_ERROR",

        "message": "Ett internt serverfel inträffade. Kontakta administratören.",

        "details": t

            {

                "code": "INVALID_VALUE_FOR_PROPERTY",

                "message": "Invalid value for property 'DefaultLanguage'."

            }

        ]

    }

}

Is it possible to have the validation also in what ever language we prefer? The ORA-messages are translated though, but it would be nice if the validation values also could be translated.