First back up the file you will be replacing, then replace the game file. Use the original local when you want to access game mails (at 10, 20, 40, ...) as the translated local will only show a blank page with no option.
You need plugin loader and signbypasser installed. An easy way is to use bnsbuddy to install this two files for you. If you want to do it manually, I can't help you, hopefully, a kind soul can give you step by step instruction.