Why exactly do we have these "interwiki" links that go to projects with no connection to WikiMedia? Why not just use templates:
{{youtube|foo}} (which might expand to [http://www.youtube.com/watch?v=%7B%7B%7B1%7D%7D%7D%7C%7B%7B%7B1%7D%7D%7D at YouTube.com]
rather than
[[YouTube:foo]]
for instance.
Why is it preferable that they be in the software itself?
Steve