Hi,
It would depend on the payment plugin. For most of them, we pass the language context so we could reload the right language when sending the email.
In the backend, when you validate an order, the email is loaded with the administrator language context.
The problem is not really an HikaShop problem, but a Joomla one.
We don't have a translation system, we use the Joomla one and it's joomla which load the language.
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.