Where are store Product Translations in database ?

Hi,

Indeed. The translations you add in HikaShop are stored in the translation override file of your website if you're not using Falang.
With Falang, it's in the #__falang_content table.

Hi,

reference_id is auto_incremented by MySQL when you insert an entry in the table. It's the id of the translation. You don't need to provide it when you add an entry to that table
language_id is the id of the language in Joomla
reference_table is the name of the target table. For a product, it should be hikashop_product
reference_field is the name of the target column in the target table. For a product description, it should be product_description
original_value is the original text to be translated.
value is the translated text
published needs to be set to 1
The rest of the columns in falang_content are not important.

First, you need to make sure your can translated a Joomla article in the different languages on your frontend. Once that's working, and the Falang driver system plugin is enabled, it will translate the content of HikaShop based on the entries in the falang_content table.