I translated wpdiscuz using PO files and loco translate plugin. The result can bee seen on the 1st attachment "before.png": I could translate almost everything except for the labels "Post Comment" and "COMMENTS".

I made two changes adding two call the __() function on the followind files:

1- /wp-content/plugins/wpdiscuz/forms/wpdFormAttr/Field/DefaultField/Submit.php - ln:100

from: <input id="wpd-field-submit-<?php echo esc_attr($uniqueId); ?>" class="wc_comm_submit wpd_not_clicked wpd-prim-button" type="submit" name="<?php echo esc_attr($name); ?>" value="<?php echo esc_attr($args["name"]); ?>" />

to: <input id="wpd-field-submit-<?php echo esc_attr($uniqueId); ?>" class="wc_comm_submit wpd_not_clicked wpd-prim-button" type="submit" name="<?php echo esc_attr($name); ?>" value="<?php echo __(esc_attr($args["name"])); ?>" />

2- /home1/cronicas/public_html/wp-content/plugins/wpdiscuz/themes/default/comment-form.php - ln:333

from: echo "<span class='wpdtc'>" . esc_html($commentsCount) . "</span> " . esc_html(1 == $commentsCount ? $form->getHeaderTextSingle() : $form->getHeaderTextPlural());

to: echo "<span class='wpdtc'>" . esc_html($commentsCount) . "</span> " . __(esc_html(1 == $commentsCount ? $form->getHeaderTextSingle() : $form->getHeaderTextPlural()));

After these changes the labels were successfully transtaled as shown int he 2nd image "after.png".

Maybe these changes can be applied on the next update or somewhere else if the texts on those points were expected to be already translated.