Try to set user.language and user.country properties in sqldeveloper.conf file located in sqldeveloper\bin folder (%APPDATA%\sqldeveloper\<version>\product.conf for more recent versions) .
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.