This is a similar if not the same issue as : MC-91132.

However, as that issue is marked as "resolved" but is still not fixed in both Minecraft versions 1.14.4 and 19w41a I decided it would be appropriate to open an issue ticket. I have only tested on Linux. I play with a bunch friends who live in Japan and as I have been studying Japanese for a while playing Minecraft together and chatting in Japanese while playing is a great learning resource....... well if it actually worked it would be.

Attempting to input chat text via IME software fails. The IME mini window pops up to show that I am indeed typing in Japanese appears but whenever I submit my input the Minecraft chat text stays blank. My friends are able to type in Japanese as it is the language their OS (All MS Windows) is installed with but it does not work in Linux.

I would also like to note, as I forgot while proofreading, that typing in another window and then copy/pasting the text does indeed work as I have a script set up to automatically do so for me binded to a hotkey but it is still super annoying to have to use that.