From where d7 render comment_form
@rakibtg, To change the comment button text you are not supposed to change any comment module which is available in the core. Simply Never hack Core!!!

For your question, to change the comment button text, 

1.You need to write up a custom module with hook_form_alter,

  function mymodule_form_comment_form_alter(&amp;$form, &amp;$form_state, $form_id) {
     $form['actions']['submit']['#value'] =&gt; t('Post Comments');
 }