In the email HTML version, near line 75 you should have the code
echo '<tr><td colspan="4" style="text-align:right">'.JText::sprintf('TOTAL_WITHOUT_VAT',$currencyHelper->format($data->cart->full_total->prices[0]->price_value,$data->order_currency_id)).'</td></tr>';
that you can replace by
echo '<tr><td colspan="4" style="text-align:right">Tax amount : '.$currencyHelper->format($data->cart->full_total->prices[0]->price_value_with_tax - $data->cart->full_total->prices[0]->price_value,$data->order_currency_id).'</td></tr>';
In the email text version, near line 30, you should have the code
echo JText::sprintf('TOTAL_WITHOUT_VAT',$currencyHelper->format($data->cart->full_total->prices[0]->price_value,$data->order_currency_id))."\n";
that you can replace by
echo 'Tax amount : '.$currencyHelper->format($data->cart->full_total->prices[0]->price_value_with_tax - $data->cart->full_total->prices[0]->price_value,$data->order_currency_id)."\n";