Edit your product_info_display template:
Replace this line
PHP Code:
<?php echo (($flag_show_product_info_quantity == 1) ? '<li>' . $products_quantity . TEXT_PRODUCT_QUANTITY . '</li>' : '') . "\n"; ?>
with this
PHP Code:
<?php echo (($flag_show_product_info_quantity == 1) ? '<li>' . $products_quantity . ($products_quantity == 1 ? TEXT_PRODUCT_QUANTITY_SINGLE : TEXT_PRODUCT_QUANTITY_PLURAL) . '</li>' : '') . "\n"; ?>
Then add the two language defines into your custom language file:
PHP Code:
define('TEXT_PRODUCT_QUANTITY_SINGLE', 'Unit in Stock');
define('TEXT_PRODUCT_QUANTITY_PLURAL', 'Units in Stock');