You don't need to assign any language to the hikashop menus. And you don't need to have joomla categories for HikaShop.
It's quite simple actually. You do your menus, hikashop categories and products in your website main language and you check that your menus display your categories/products.
Then, make sure that the content edition is activated in the Languages tab of the configuration of HikaShop.
Then, edit your products/categories and you will have flags on which you can click to translate them in the other languages of your website.
Once that's done, the translations of your content will display automatically thanks to falang based on the current language.
You should not remove falang.