Google services are blocked in mainland China, but there are still millions of Chinese speakers across the world with access to Google Assistant. Support for Traditional Chinese was added to Assistant last year, and now it works with Simplified Chinese too.
The official support page for Assistant languages says Simplified Chinese is currently only supported on phones — smart displays, smart speakers, smartwatches, and other form factors will have to wait.
It's important to note that Simplified/Traditional Chinese only refers to text displayed on screen, and it's not clear if anything other than Mandarin for spoken word is supported.
- Google Assistant Help