A component milestone of Wikidata/WiktionaryZ development is true multi-language support for page titles and content on the MediaWiki level. We have funding to pay a developer to implement it (I'll be working on versioning and object/relational mapping in the meantime), and I have written the first set of specifications for this:
http://meta.wikimedia.org/wiki/Multilingual_MediaWiki
Note that this is in no way a proposal to merge existing Wikimedia projects; in fact, MediaWiki should be fully backwards compatible and continue to act exactly as it does now in monolingual installations (there'll be a couple of new features, such as persistent UI language selection for anonymous users). However, multilanguage support should be enabled for wikis like Commons and Meta, where multiple languages are handled in a single database.
I think I've come up with a fairly clever way of dealing with the community issues of language filtering, but see for yourself. In any event, I'd much appreciate feedback on this proposal before we go ahead with the implementation, especially from Brion as the release manager. :-)
Best,
Erik