You can do so by modifying qa-lang-question.php that's inside public_html/qa-include/lang/ in your FTP
Outer button
'comment_button' => 'comment',
Inner button
'add_comment_button' => 'Add comment',
If you use Q2A in another language, you'll find qa-lang-question.php file inside language folders in public_html/qa-lang directory.
I hope it helps!