Hello,
There is a "loadLocale" function in HikaMarket mail class ; that function derives of the one in the HikaShop mail class.
But, during the tests when I implemented the feature, it generated some unwanted side effects and could have conflicts between HikaShop and HikaMarket (which were both trying to change the language context).
Because it's not something really handle by Joomla (to change the language context within the page generation) ; I can only say that's a sensible part.
I hope that we would find a good way to change the Joomla language context easily, without creating side effect or language errors (because when you change the language you need to handle the overrides too).
Afterwards, a vendor do not have a language ; only a Joomla account have.
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.