It's up to you how you customize the codes. For doing it properly, you should document your changes. It's quite a lame approach to apply custom changes to the open source program, which has to be updated in the future.
As about the alert message, I'm not sure what you are about. If you leave the topic title empty, the script will alert "Please, enter Subject". If you do not fill the message fill, it will alert "Please, enter Message". If you do not fill in both fields, both messages will be displayed. It works and could be tested even here on our board.
If you want to change the language wording itself, here is How to find where to edit this and that
. Look up for a phrase which you want to change, and you will find the script where to change it.