How can I use cloud-based text-to-speech services as native voices in Mac OS?
https://cloud.google.com/text-to-speech/docs/quickstart-protocol
This article lists all the steps required.


Enabling the service's trial in Google Account.
Installing SDK


^ This is one time setup. The following is to be repeated.


Make the curl command that a shell can execute. This has three parts, two of which are fixed: voice type and audio config. The third one, input needs to be changed. *
Get the JSON response and save it to text file.
Decode the text file, to mp3 using  

base64 synthesize-output-base64.txt --decode &gt; synthesized-audio.mp3

Play it.
Bind all of this in a shortcut for the preferred app as a service. 


Most of your shell task can be done via "run shell script" in automator. 

* For changing the text part, you can find multiple questions, or even ask one to know how to get selected text in an automator variable. Another option would be to copy paste text in an Automator's app's popup.

Next, put that text variable in the shell script. 

Then make a dedicated folder for all the text and audio files that will be made in an action. Save the received JSON response there. The command to decode it will be fixed, since the file location and name is the same. 


All of this can be put in an Automator app which displays a popup that has a text field and a Submit/ Play button.