Synopsis

Chatting in Minecraft is an essential feature for rich multiplayer experience. I believe that the core part of Minecraft gameplay is online collaboration with friends and world-wide players. To this end, chatting in our native languages significantly lowers the learning curve for beginners and boosts fun for experienced players.

In MC-2781, many users from China, Japan, Korea, and a few western countries have reported CJK IME issues and in 15w43a Mojang has applied a "quick" patch that accepts characters generated by operating system IME.
This "quick" patch is not perfect, however, with the following issues:

• It works only with Windows. On Mac and Linux, the IME behavior is different and inconsistent with different IME plug-ins.
• It does not disable IME when the chat window is closed, blocking in-game control keys (e.g., WASD) if the user does not explicitly change to English input mode.

Suggested Complete Solution

Edit Please see comment from July 3, 2020 to see solution information.

• Send a patch to LWJGL to add a cross-platform IME API like SDL. https://wiki.libsdl.org/Tutorials/TextInput
• Update LWJGL of Minecraft.
• Patch Minecraft to:
  • disable IME by default
  • enable IME when the chat window / sign input window opens.
  • disable IME when the chat window / sign input window closes.
  • accept character composition events to actively update texts in the input window for native experience (so-called "on-the-spot" IME)

This will be a long-term journey because we need to patch both LWJGL and Minecraft whose authorities are different, so I create another issue to clean up long discussions and suggestions in the original issue (MC-2781).
I think the community users and Mojang should work together to completely resolve this issue.