What's the use case? Currently Translate extension stores different translation pages in different page language (page language is a core feature), and is it enough to get the language there?

BTW: Page language can be changed on-wiki via a special page.

It should be possible to

• Link source page of translatable page to Wikibase repository
• Wikibase repository automatically fetch, link and update translated pages of the source translatable page by [Translate + Wikibase client]
• Wikibase client send updates for translated pages (move, deletion, etc.)