Google Assistant's language support is beyond confusing. I've been covering it for over 6 months now and I still don't understand why a language could work in one version of Assistant but not another. Take Hindi for example. It works in Allo, but not in other instances of Assistant like the main one on your phone. That's changing though now.
If you have your phone's language set to English - India, you'll be able to activate Assistant by tapping and holding on the Home button on any Android 5.0 and above phone (tablets probably won't work as they only support US English for now).Read More
Google Assistant now understands Hindi, Actions on Google can be developed in Russian (ru-RU) was written by the awesome team at Android Police.