Windows 10 users who run Insider Builds can test the new functionality already; the change is live in the most recent Windows 10 version 1903 Insider Build and in the Skip Ahead build as well.
As with most of Google's products, Assistant is an incredibly powerful tool in the United States, but its functionality is limited in other countries. This is understandable, since there are dozens of other major languages worldwide with countless dialects, and speech recognition for each variation can take a while to develop. At Mobile World Congress, Google announced a massive expansion for Assistant's language support.
After the shutdown of Allo, it was only a matter of time before Google took the messaging app's few good ideas and implemented them in its Messages platform. The most important of these, Assistant's integration in chats, was inevitably going to make the jump, and today Google confirmed that the feature will go live in Messages over the coming months for English users.