Hi,
The Joomla translation system is allow any component to translate or override translations.
When HikaShop component is loaded, his translations (and translation overrides) are loaded too.
The fact that your override is loaded and used does not means that you're using HikaShop ; but it means that there is a plugin which has loaded HikaShop.
And yes, you should use the plugin "Redirect Joomla registration to HikaShop Plugin" to have the HikaShop registration form instead of the Joomla registration form. Just enable it via the Joomla plugin manager. Note that you might need to set the menu item id setting of the plugin with the id of a menu item of the HikaShop component if you get a 404 error when you enable it.
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.