Got it working (well sort of). In the Google Home app on the phone, set the Language to English US (for both the Home and your phone). Then "Hey Google, Connect to Rachio" and Google Home will send a "Card" to your Google Home app with a "Link" button. Follow the prompts. It still does not show up in the Google Home app "Devices" tab but you can now "Hey Google, Talk to Rachio" and GH will respond with my account name and ask me for an action. "Ask Rachio to Water all Zones" / "Ask Rachio to Stop" worked! (I did have trouble initially getting actions to work due to the syntax needed).
Still, turns out it is just easier to use the Rachio App as I have 2 Rachio's and the Integration with Google Home only support one at a time (you have to do the Link thing all over to change between Rachios).
Anyway... it looks like Rachio need to authorise the other Lang Support like English (Australian) for it to appear and respond correctly.