We can type multibyte characters on the Windows version, but not on macOS. I'm a Chinese user and even when I switch to Chinese input they still type out as english in the game chat window.
We have had community support for the older version of Minecraft (https://minecraft.curseforge.com/projects/inputfix) but they no longer work on the new version and their support on Linux/MacOS is very limited.
This is MC-91132