Try to set user.language and user.country properties in sqldeveloper.conf file located in sqldeveloper\bin\ folder.
Sample for set french language:
IncludeConfFile ../../ide/bin/ide.conf
SetJavaHome D:\jdk1.6.0_25
AddVMOption -Doracle.ide.util.AddinPolicyUtils.OVERRIDE_FLAG=true
AddVMOption -Dsun.java2d.ddoffscreen=false
AddVMOption -Dwindows.shell.font.languages=
AddVMOption -Duser.language=fr
AddVMOption -Duser.country=FR
AddVMOption -XX:MaxPermSize=128M
AddVMOption -Doracle.jdbc.mapDateToTimestamp=false
IncludeConfFile sqldeveloper-nondebug.conf
To switch off language try to remove its support by editing oracle.translated.locales option in ide.boot file (located in ide\bin folder).
For example, if I want to ‘break’ french language support this option will be:
oracle.translated.locales = de,es,it,ja,ko,pt_BR,zh_CN,zh_TW
The original value was:
oracle.translated.locales = de,fr,es,it,ja,ko,pt_BR,zh_CN,zh_TW
The configuration above (sqldeveloper.conf file) will be wrong after remove french support and the french language does not appear as GUI localization.
You can also set language at runtime
sqldeveloper.exe --AddVMOption=-Duser.language=en