All REST API queries require a valid X-Cart access token.
You can find access tokens (one for read only requests and one for read/write requests) in the API section of your store's Admin area (Settings -> API).
Be sure to include your token as an X-AUTH-TOKEN header on all API queries.
X-Cart REST API supports multiple languages, but all the requests are made in the same language. The language is set using the Accept-Language request HTTP header https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept-Language
If the header has not been sent, the default language as specified in the store settings is used.