How do i made wordpress comment form custom fields placeholders available for both login and logout users

Hi, there.

I am facing a little but serious problem in wordpress’s comment form.
I want to change the placeholders of comment box(textarea) and submit button value.

i tried changing in comment-templates.php file in wp-includes folder and it works for me only for login users but not work for logout user.

I appriciate you help.
Thanks