After saying the wake words "Ok, Google," users can use Spanish to speak with Assistant on the Home, Mini and Max. To hear about your schedule and appointments for the day, you can say, "¿Cómo será mi día?" Or to turn down the temperature, simply tell the Assistant, "Baja la temperatura del termostato."
These features are available today on the afore mentioned Google Home hardware. If you're using Google Assistant on Android, the service has been available in Spanish to speakers in the US, Mexico and Spain since last November.