If you use the
Translation API Client Library the original input value is returned within the translated response. Naturally, if you are running the API call on a 'source' string, you already have the original reverted translation.
In the
REST API example URL you show a translation from English to English which will of course produce a warning as this is not a valid translation language pair. By changing the 'target' parameter from 'en' to 'hi' in the URL, it will produce the source English text into Hindi.
Note that you are using the Standard Edition of the API instead of the
Premium Edition where the translation can use the newer
'nmt' model, and will return the 'detectedSourceLanguage', which you can use to set as the 'target' language in another REST call to get the reverse translation you want.