Hi,
Which version of HikaShop are you using ? (and with which Joomla version ?)
In HikaShop 2.0 we made some modifications in order to load the override language file even if we are in the backend. So it should fix the problem.
Otherwise, you could have trouble to load the override file in the backend, so, some email could be not translated correctly if they are generated from the backend.
If the problem is not solved, the best way would be to not use the override for theses translations but to edit directly your language file.
I know that's not a perfect solution because if you update your language file you would loose your modifications but, it could fix your problem if HikaShop 2.0 does not.
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.