I found the solution in another topic :
Here is how to do :
– edit the theme-comments.php file
– find the section begginning with “$args = array”
– add these 2 lines at the end of the section (be careful with the commas at the end of the lines)
'submit_button' => '<input name="%1$s" type="submit" id="%2$s" class="%3$s" value="%4$s" />',
'submit_field' => '<p class="form-submit">%1$s %2$s</p>'