The data format is determined by the HTTP request headers Content-Type, Accept, Accept-Charset, and Accept-Language.
These are the headers to send and receive JSON:
Content-Type: application/json; charset=utf-8;
The Content-Type header is only needed in POST or PUT requests, since the body is not evaluated in a GET request.
The request is expected in UTF-8, and also the response uses this encoding. The charset header is optional.
The Accept-Language does not influence the payment window but the API response, for instance error messages. It can be set to de_DE or en_US. The language header is optional, German is the default.