In a recent comment, @laurel wrote
I think it's helpful to link to relevant questions on other sites. In fact, I think it should be taken a step further, with "association banners" like some questions on SO have to Ru.SO questions. (Due to the language difference, these banners are only visible to people with certain browser settings. I'm not sure if there are any plans to bring this feature to the rest of the network unfortunately.)
ColleenV, the esteemed moderator of our sister site, English Language Learners, dug up an old SE Meta question proposing
Display other SE site links in the linked questions sidebar
The feature request was declined, with the following reasoning by Jon Ericson:
I suggested this feature to the dev team. It turns out the linkback feature makes it difficult to implement and expensive to put in operation. Remember, we have 133 (and counting) sites with millions of questions. If a crosslink is added or removed anywhere on those sites, the sidebar on both questions must be updated.
It might possible to optimize this and make it reasonably performant. But remember that these links would only appear on a tiny fraction of questions. So while we think it's an interesting feature, it's not one we will be investing in anytime soon.
Colleen and I have collaborated to try to resuscitate the proposal, Colleen with a comment below Jon's answer, and I with a small bounty on the question. (Both of these tactics were suggested by someone in a comment.