According to Android Police, to use Assistant in Japanese, it must be the only language you've enabled -- you'll have to disable any other language on your device. Don't worry, though: you can still activate the voice AI simply by saying "OK, Google" even if you switch to Japanese. It certainly took Google quite a while to introduce another language for the improved Assistant experience, which boasts deeper integration and improved speech processing among other features. We could only hope that the company is working on adding support for more languages, and that Japanese is just the first of many it's planning to roll out.