I'm also not sure of the criteria for adding interwiki links.

Given the status of Wikipedia as "the free encyclopædia that anyone can edit", I think it boils down to "someone thought it was a good idea at the time".

Ideally, I assume they're supposed to go to "the equivalent article" in the other language, but there's not always a 1:1 equivalence, so finding the best article to link to is not always obvious.