I am using ImageJ 1.50i on Ubuntu with Fcitx Chinese Input Method. The problem is that Chinese character can’t either be added directly by the input method or be pasted into Text box from other editor. The input method can’t be called out in both Text selection on image and Text Window from Plugins Menu. But the characters can be pasted into Text Window, so there is a indirect solution to add them by running Macro
drawString("中文", 10, 80).
The typing problem is same for CJK characters, I guess.