This thread is resolved. Here is a description of the problem and solution.
Related products don't appear on secondary language.
Setting the custom field '_wcrp_related_ids' to 'translate' solves the issue.
-On WPML > Settings > Custom Fields Translation, click 'Show system fields' -search for the custom field '_wcrp_related_ids' and set it to 'Translate' -Save. -edit the product. edit it on the original language. -translate the product. You should have the ID field in the WCML editor to add the ID of the translated related product.
This is the technical support forum for WPML - the multilingual WordPress plugin.