Tested with merlin-beta b52, b53 on Solaris 2.6, 7, 8sparc, Solaris 8 x86 on ja locale.

JTextPane has a problem in Japanese input mode with Java L&F.
In case of Java L&F, making Japanese input mode available and entering Japanese characters, Japanese characters can be displayed without any problem for the fist time.
Then, Japanese input mode being kept, if one of the JMenu of the application is clicked, Motif's Japanese input window appears and Japanese characters can not be entered any more.

To reproduce,

1. Launch StylePad
  ($JAVAHOME/demo/jfc/Stylepad/Stylepad.jar)
2. Press "ctrl + space" to make Japanese input mode available.
3. Enter some Japanese characters.
4. Click one of the Menu title.
Then, you can see Motif's Japanese input window appears at the bottom of the application.
Entering any characters can not be accepted (though non-Japanese characters can sometims be accepted).

or
for step 4, if you click another window and go back onto Stylepad instaed of clicking Stylepad's menu, the same problem occrs.