So it's time I contributed something to this magnificent project. I have made some code changes to make the invoice multilingual.

in YOUR_ADMIN_FOLDER\includes\super_invoice find rule 268

PHP Code:
<?php
  
for ($i 0$n sizeof($order->totals); $i $n$i++) {
  
$display_title $order->totals[$i]['title'];

    echo 
'          <tr>' "\n" .
         
'            <td align="right" class="'str_replace('_''-'$order->totals[$i]['class']) . '-Text">' $display_title '</td>' "\n" .
         
'            <td align="right" class="'str_replace('_''-'$order->totals[$i]['class']) . '-Amount">' $order->totals[$i]['text'] . '</td>' "\n" .
         
'          </tr>' "\n";
    
$order_total $order->totals[$i]['text'];
  }

    echo 
'          <tr>' "\n" .
         
'            <td class="dataTableContent" align="right" valign="top"><strong>Amount Paid:</strong></td>' "\n" .
         
'            <td class="dataTableContent" align="right" valign="top"><strong>' $currencies->format($so->amount_applied) . '</strong></td>' "\n" .
         
'          </tr>' "\n";

    echo 
'          <tr>' "\n" .
         
'            <td class="dataTableContent" align="right" valign="top"><strong>Balance Due:</strong></td>' "\n" .
         
'            <td class="dataTableContent" align="right" valign="top"><strong>' $currencies->format($so->balance_due) . '</strong></td>' "\n" .
         
'          </tr>' "\n";
?>
and replace with:
PHP Code:
<?php
  
for ($i 0$n sizeof($order->totals); $i $n$i++) {
  
$display_title $order->totals[$i]['title'];

    echo 
'          <tr>' "\n" .
         
'            <td align="right" class="'str_replace('_''-'$order->totals[$i]['class']) . '-Text">' $display_title '</td>' "\n" .
         
'            <td align="right" class="'str_replace('_''-'$order->totals[$i]['class']) . '-Amount">' $order->totals[$i]['text'] . '</td>' "\n" .
         
'          </tr>' "\n";
    
$order_total $order->totals[$i]['text'];
  }

    echo 
'          <tr>' "\n" .
         
'            <td class="dataTableContent" align="right" valign="top"><strong>'ENTRY_AMOUNT_APPLIED.'</strong></td>' "\n" .
         
'            <td class="dataTableContent" align="right" valign="top"><strong>' $currencies->format($so->amount_applied) . '</strong></td>' "\n" .
         
'          </tr>' "\n";

    echo 
'          <tr>' "\n" .
         
'            <td class="dataTableContent" align="right" valign="top"><strong>'ENTRY_BALANCE_DUE.'</strong></td>' "\n" .
         
'            <td class="dataTableContent" align="right" valign="top"><strong>' $currencies->format($so->balance_due) . '</strong></td>' "\n" .
         
'          </tr>' "\n";
?>
In the language file YOUR_ADMIN_FOLDER\includes\languages\YOUR_LANGUAGE\super_invoice.php add two defines

PHP Code:
define('ENTRY_AMOUNT_APPLIED''Amount Applied:');
define('ENTRY_BALANCE_DUE''Balance Due:');