Related product is using the link information of the product it's linked to - which can be translated separately under "Related products Link image" section.

Please follow the outlined procedures below.

- For the main language, edit the Product with related products – in my example below, I have 2 related products in “Product 6”

- Then change the Related Product TITLE and DESCRIPTION

- To update the “Related product” translations (in my example above, it’s “Product 7” that was set as “Related Product” in Product 6,  click “Translate Content” ->  click EDIT button to translate the “Related products Link image” for the cross-promoted product

- The changes take effect.

The main language must be edited in Normal mode and other languages in Translate content – as per steps above.