Tested on Linux Redhat 6.2. JDK:
java -version:
java version "1.3.1"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.1-b24)
Java HotSpot(TM) Client VM (build 1.3.1-b24, mixed mode)

I set czech keyboard via command:
setxkbmap -symbols czsk(us_cz_qwerty)
and runned Forte for Java (3.0) with "cs_CZ" locale (I also runned simply JEditorPane application, the results were the same).

I was able to use national characters, but when I set "numlock" to true, I was not able to use <Enter> key under Java (that means inserting new line). It was OK in native applications (gEdit and vim), but my test application reports that any enter produces Alt+Enter keystroke.