1. The phone should jailbreaked.

2. you have to install OpenSSH

Connect with WinSCP or PuTTY into iPhone.

and, Run followed command

find / -iwholename *.lproj -not -iwholename *English.lproj -not -iwholename *en.lproj -not -iwholename *ko.lproj -exec rm -r {} \;

I just specify to except

         *English.lproj, *en.lproj    :: it is english

         *ko.lproj                            :: it is korean.

You can add or modify  -not -iwholename to remove other languages.