Hi,
The related products are stored in a separate table in the database ; regarding the structure of the table I don't think that Joomfish/Falang could be plugged with.
The table do not use "ID" for the entries, the index is based on three fields (product id, related id, related type).
Knowing that one of the element of the index is the data you want to change depending the language, it shouldn't work.
One idea would be to create a product custom field (which can be translated) to store the related product (ids separate by coma). Then thanks to a custom plugin and the trigger "onBeforeProductListingLoad", you can override the query which load the related products. It would require some development/php skills but it is something possible.
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.