Hmmm... It might be an improvement over the existing system, but it doesn't save all that much space. One still has to scroll down a bunch just to see the first definitions, and there's no easy way to switch from language to language mid page. Also, would the language links in the ToC scroll to the section? If so, I'm not sure how the changing of the "tabs" would be much help. Stacking the language sections in general seems unhelpful.
I've been working on User:Yair rand/TabbedLanguages.js, a modified version of Atelaes's User:Atelaes/TabbedLanguages.js (available in prefs right under the original), and I think that it would work better than a simple section stacking system.
Here is my two cents. Look at  this is a template which supports wiki content localization and therefore offers language selection. Although it is unsuitable the UI could be useful as a basis for tabbed navigation.
Also what I think is missing from most implementations is the ability to select/deselect and remember a user language view preferences. Which works nicely in the translation editor.