I wanted to implement this sortable views on a list of translated entities and make sure that the individual translated nodes had different orderings.
Therefore, my patch updates the postSave() method by checking whether there is an existing node, and if there is then it will save the translated version of the node.
Needs work
Code
The change is currently missing an automated test that fails when run with the original code, and succeeds when the bug has been fixed.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Ideally new tests would be added that make sure translations are saved separately.
Having a view listing multiple translations of the same node would also cause issues, since only a list of ID's is sent to /sortableviews/ajax. Ideally, that would be a list of ID/langcode combinations. Let's fix that here as well.