Hi,
The content of this email in store in two views called "show_email_notification".
One for the backend and one for the front-end (it depends "who" will validate the order).
Did you override one of these views ?
HikaSerial have translations for SERIAL_DATA and ATTACHED_TO_PRODUCT since version 1.0
ATTACHED_TO_PRODUCT="Attached to product"
SERIAL_DATA="Serial Data"
If your language file have an error it could be possible that HikaSerial translation wouldn't be loaded by Joomla.
The override file is used by HikaSerial and HikaShop so, evenif HikaSerial does not load properly his language file, HikaShop could use the override file.
Regards,