At launch, the new Google Assistant on the Pixel 4 came with several limitations. Beside requiring the Pixel 4, it didn't work for those who have a G Suite account on their phone, required gesture navigation, and didn't support multiple languages, but worse yet, it was limited to US English only. With time, some of these obstacles were removed, like the navigation requirement, and we've seen the addition of more English variants. Now, the new Assistant is going beyond English for the first time, with official support for Japanese.
While Japanese isn't the first option we would have expected to see, it seems that Google decided to bundle the Assistant update with the rollout of Motion Sense to the Pixel 4 in Japan, a country where the feature was previously disabled.
Back to Assistant though, if the only language you've enabled is Japanese and you trigger Assistant, you should get the new minimal UI with more contextual features, continuous conversation, and plenty of cool tricks and special Chrome abilities. This ability is rolling out starting today, but it won't work if you have multiple languages (even if the second one is English).
Hopefully, this will be the start of a wider rollout of the new Assistant with support for more languages and locales.