At the moment, if you you post just a link to a web site, such as Wikipedia, it will be 'oneboxed'. It would be nice if it could be extended so you could write something like
and have that article linked to.
I agree that to allow for more constant syntax, it should be wrapped in
With regards to language, seeming as SO is primarily an English speaking community, I see no reason to consider other languages. If we had say a Spanish SO, then this could just make use of the Spanish Wikipedia.
As to handling articles that do no exist, perhaps this could be done client side. so if you tried to link to a non existent article, it will inform you. Perhaps offering a link to the search results, perhaps simple linking the 'create a page' page for Wikipedia.
I am sure that if this basic frame work could be implemented, it could be extended to for use with other sites as well.