I bump this.
http://www.minecraftwiki.net/wiki/Known_bugs/Version_1.3.1#Language.2FText_FilesMinecraft is implemented on top of LWJGL and suffers from the lack of IME despite of its almost full localization contributed by the community.
Some issues should be fixed in the Minecraft part, but I think coordination in the LWJGL side is also required.
Since I'm not an expert on Java-based game development, I post the same issue on both Minecraft and LWJGL.
Please separate the issues correctly for each part, and fix them.