Go to Netbeans installation directory, for example,

C:\\Program Files\\NetBeans 6.0.1\\etc

Open "netbeans.conf" and find netbeans default option line

netbeans_default_options="-J-client -J-Xss2m -J-Xms32m -J-XX:PermSize=32m -J-XX:MaxPermSize=200m -J-Xverify:none -J-Dapple.laf.useScreenMenuBar=true"

Add "-J-Duser.language=en -J-Duser.region=US" to the end of this line

netbeans_default_options="-J-client -J-Xss2m -J-Xms32m -J-XX:PermSize=32m -J-XX:MaxPermSize=200m -J-Xverify:none -J-Dapple.laf.useScreenMenuBar=true -J-Duser.language=en -J-Duser.region=US"