That API was deprecated seven years ago!
The Google Transliterate API has been officially deprecated as of May 26, 2011.
The deprecation policy
] says they'll do their best to keep it running for up to three years from that date. After that, they can withdraw the API at any time, without notice.
be able to use the upgrade-insecure-requests
] CSP to force the script to load over HTTPS. But there's no guarantee that it will work, and a very real possibility that Google will terminate the API without notice and break your site.
You'll need to find an alternative API to meet your requirements.