SCIM is a Smart Common Input Method platform which provides a 
user-friendly and full-featured input method user interface for 
POSIX-style operating systems.

Currently the SCIM project has a wide range of input methods 
(some may need other libraries) covering more than 30 languages, 
including (Simplified/Traditional) Chinese, Japanese, Korean,
 and many European languages.  Composing/Dead key support is 
also one of the built-in features.  In addition, several projects 
have been established to design IMEngines for SCIM, and others 
supply their own SCIM plugins.

To use scim, you need set something in ~/.xinitrc like:
    export GTK_IM_MODULE="xim"
    export XMODIFIERS="@im=SCIM"
then start scim:
    scim -d 1> /dev/null 2> /dev/null

This is just a platform; you also need a special input method.
I used scim-fcitx for Chinese PinYin input. 
I also will submit scim-fcitx to SlackBuilds.org later.