With the latest nightly builds of LWJGL, users on Mac/OS X should be able to type any Unicode characters (including Chinese, Japanese and Korean).
This issue is fixed with the latest nightly builds of LWJGL, so will be fixed as soon as Minecraft is bundled with the newest version of LWJGL.
This is fixed in the nightly builds of LWJGL and will be part of the LWJGL 2.9.1 release (in the mean time you can use the nightly builds of LWJGL). Therefore this bug will be fixed in Minecraft as soon as they upgrade the LWJGL version.