I made a Japanese input software (Input Method Editor: IME) of public domain for Windows 2000. It should work on ReactOS without any porting.
Japanese inputs require text conversion because it uses the mixture of Hiragana, Katakana, and the Chinese-oriented characters (called as Kanji). Normally the Japanese user types the "reading" characters in Hiragana, and then convert them into the mixture (composition). A Japanese IME works as a text processor between the user and the GUI system.
Documentation About IME/IMM: https://katahiromz.web.fc2.com/colony3rd/imehackerz/en/
I think ReactOS needs to be fixed on:
- Registering/Unregistering IMEs
- The WM_IME_* messages
- Behavior of The CS_IME window class style
- The EDIT control