Start by formalising this concept, and producing a special interface for people who are reading the non-local version of their Wikipedia. For example, a user in the .fr domain whose language settings are set to FR reads a page on EN. The interface could simultaneously present him with the english and french versions of the article (assuming they exist) and perhaps suggest to him that he might like to contribute to the FR version.
I like the way you're thinking, but I'm not sure I like your idea. How about (once single sign-on is set up), a global setting where you can put (in order) your preferred languages. Then, whenever you click on a link, you can have it set to either:
1) Take you to the article in the language which is highest on your list of preferred languages out of the languages that have an article on that subject (judged by the presence of interwiki links) 2) Take you to the article in the same language as the article you were reading, but with a very noticeable message at the top saying "This article also exists in LANGUAGE" if there is an article in a language you prefer. It should also say if another of your preferred languages has it as a featured article. 3) Do exactly what it does now, for those that don't like change.
If you are reading an article in one language, and another of your preferred languages doesn't have an article on that topic, it would be great if, next to "Edit this page" was a "Create this article in LANGUAGE" link - although if you clicked it, it would need to suggest you look for an article in that language that simply isn't linked and link it, before starting from scratch.