Google Cloud Translate API - Does en_IN not support en yet?

[sph-yulee]

https://translation.googleapis.com/language/translate/v2
-d '{"target":"en_IN","source":"en","format":"html","model":"base","q":"Free Tournament Ticket!","q":"Congratulations!"}'

Upper request return error like below.
{
  "error": {
    "code": 400,
    "message": "Bad language pair: {0}",
    "errors": [
      {
        "message": "Bad language pair: {0}",
        "domain": "global",
        "reason": "badRequest"
      }
    ]
  }
}

But if source is "ko" or "zh_CN", it doesn't return error.
ex) https://translation.googleapis.com/language/translate/v2
-d '{"target":"en_IN","source":"ko","format":"html","model":"base","q":"안녕!","q":"축하!"}'

For now, en_IN is not on the support language list, (https://cloud.google.com/translate/docs/languages?hl=ko).
Should we wait until en_IN is on the supported language list,
to be used in "target" with other supported language?