Guaka:
Would it be feasible to allow something like preload=de:Berlin to get articles from other languages, or even other Wikimedia projects?
Other languages are, technically speaking, separate projects, so that's basically the same thing. Kate is currently working on interwiki transclusion, perhaps she might be interested in pursuing this.
gives you a modified introduction, which is useful when creating custom edit pages. The two parameters only work when the page does not exist yet.
This only shows when the article doesn't exist yet. Is that a feature?
It's a known limitation. It should probably be shown on all pages.
Erik