Hi,
I also think it is possible to do that with Falang, and the website of your example does use Falang there.
However, I'm not sure why pouce says it doesn't work with Falang. That's something you would have to check with Falang's support if it doesn't work properly, because as I said before, the switch is not done by HikaShop but by the switcher module.
And the fact that it works with the Joomla articles associations is just because joomla has some extra code in the switching system to handle that.