Daniel-
What about the other Wikimedia projects (not to mention all the other non-Wikimedia projects that use MediaWiki)?
That's not the issue. We can proceed as follows:
1) Each Wikipedia has an editable Help: namespace, where their language handbook is maintained;
2) The other Wikimedia projects, and in fact any MediaWiki site, can import a dump of the help pages from the respective Wikipedia in their language (dumping and importing just the help: pages is trivial), as protected pages. If they want to make changes, they are encouraged, but not required, to make them to the Wikipedia version. During the next update they can choose to just update unmodified pages.
The only thing we need to take care of in this scenario is avoiding Wikipedia references and links from help texts to policy pages (still abundant through much of our current help texts).
That is why I started the handbook on Meta in the first place!
I would love to have the handbook on Meta. Before we do something hasty I'd like to think this through a bit more. My original idea was to have Help: namespaces in various languages on Meta: - Help: / Help talk: - Hilfe: / Hilfe Diskussion: - Aide: / Aide discuter: ..
This would be reasonably easy if we switch Meta to a separate codebase from Wikipedia (like test.wikipedia.org's codebase is separate), or if we beefed up the namespace system a little to allow for easy expansion of namespaces. In the current setup (shared codebase with all wikis) it is nontrivial.
As for the user interface, given that the kind of people who work on the handbook need to get a lot of info from the English version anyway, I think having an English user interface for maintaining the documentation is not such a big deal, at least in the short term.
Would it be possible to have development on the handbook occur on Meta MediaWiki pages (at least for English)
There's no reason to do that. The MediaWiki pages are for MediaWiki interface strings only. User-created custom messages which are currently in the MediaWiki: namespace will be in a Template: namespace in the next version of the software. Exportability or importability is not the issue as soon as all pages belonging to the set you are interested in (in our case, help pages) can be clearly identified. Even if all pages just started with "MediaWiki Users' Guide", we could export and import just these pages, but a real namespace makes things a lot more maintainable and reliable.
If your concern is keeping the handbooks in sync, and you imagine a kind of interwiki transclusion system, that is not really the problem. As noted above, wherever the help pages live, they will be unprotected. Wherever they do not live, they will be protected. We can even make the Edit page point somewhere else, although that might be confusing, we can put a special notice on all Help: pages that are not "live" etc. - all kinds of possibilities. Updating within regular intervals is trivial.
Regards,
Erik