Setting aside the technical difficulties of this for a moment, given that we don't actually run the interwiki module, I have a number of objections to this policy, especially in terms of "Celebrating Diversity".
There's the obvious problem in that this is setting different forms of diversity against one another - do we include our Spanish-speaking authors and readers, interested in reading EN works in their own language when available, or do we allow authors to exclude them in order to make a point about the Spanish-speaking branch's policies? I really don't think that not displaying its translations on your pages is the best way to promote a change in ES policy - that would be up to the staff and users of ES, who I assume have a range of opinions on this, and this doesn't seem like a very productive way to engage with them.
More broadly, I like having the Interwiki module be a tool that's outside of the control of any one author or branch (in principle, even if in reality it's run by one Russian). Allowing the option of excluding specific branches would enable many things we might find problematic, ranging from a branch making a political statement by blocking another branch's translations from its version of the Interwiki module (which can actually be done, I'd rather not encourage it though), to me excluding German translations from pages out of a personal dislike of the Germans (joking, I love the Germans). Enabling individual authors to chose which translations are included would open up a can of worms that I'd rather keep closed, given the difficulty of resolving disputes between branches, especially when it would likely touch on national politics. Right now, the Interwiki module is just a thing that you opt into when posting on any of the SCP Foundation sites - much like the sidebar, you just have to accept that the links on it aren't something you control. The translations listed there could be really bad, or completely out of date - that's an issue for people on the other branch to resolve.