Hi,
This is not handled by HikaShop but by the language switcher module you're using.
Note that you have two language switcher: Joomla's and Falang's.
Since you're using Falang, make sure that you're using Falang's language switcher and not Joomla's.
Joomla's language switcher module doesn't support staying on the same page of a menu item, and it directs you to the main page of the current menu item, which corresponds to what you're describing.
If you have the issue with Falang's language switcher module, I would recommend to first check with Falang's support. They should be able to better help you than us as we don't know how their language switcher works.