I will like to start saying a very big thank you to @bggold who asked me to consider this in the first place
While there isn't any way to use the nice Google Assistant voice, and it's still rough around the edges, this demonstrates how we can use Android Auto in a multi-lingual environment as well as how to read out and reply emails.
The concept is very simple, instead of relaying on Android Auto to read out the notifications I have created a small app which mimics that behaviour, with some extras:
1) It try to do a language detection for each test, and use the appropriate TextToSpeech
2) It allows the user to select which app notification does he/she want's to read
3) If the content is really long (like a lengthy email) the app will prompt the user if it want's to go ahead with the message readout or not
4) If notification supports quick-reply the app will prompt the user for a reply (in the same language as the the original message)
Planning to support a lot of languages but there I will need user input, I have translated the basic strings to around 70 languages using Google translator, but I can tell it's not quite right (I can see with the languages I do speak)
Best part, app won't be affected by 3rd party restriction at all and it won't require root either, however I'm pretty sure Google will not allow it on the PlayStore
Beta version available now on XDA Labs.
App will be payable, however current beta is free so grab a copy while you can
To use the app you will need to enable unknown sources in Android Auto, simply open Android Auto, go to About, tap the "About Android Auto" header 10 times till you become a developer, then from the 3 dotted right menu select developer options and enable "Unknown sources"