In the sample code, above, I hard-coded that text. If you want it picked up from the language file, you would need to add a new entry for it in both language files and use a constant like this:
<tr>
<td class="field-title" align="left" valign="top"><strong><?php echo INV_AUTHNET_AMOUNT_TO_PAY; ?></strong></td>
<td class="field-title" align="left" valign="top"><strong>$<?php echo format_number($params['x_amount'], 2); ?></strong></td>
</tr>
Then in the authnet_english.php file, add this to the end (before the ?> close tag, if applicable):
define("INV_AUTHNET_AMOUNT_TO_PAY", "Amount to Pay");
...and an equivalent line in the French file.