Hi,
As long as the customer does not have any Joomla account ; that account won't have any language (which is managed by Joomla itself).
So when the email are send to the customer, the default website language or the current language (if you're in the backend) will be used.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.