breaking up MediaWiki message group to smaller projects
You should have told him to start on the appoximately 500 "most used MediaWiki messages" group ;-) that certainly looks less intimidating. When I was done with that group, the (at that time) remaining 1,700 or 1,800 MediaWiki core messages looked very doable to me :-)
The 500 are already translated into his language :)
And 1,700 is still quite a lot.
It's not just about intimidation, it's also about focus. Well-defined groups like "editor", "navigation bar", "preferences", "special pages", "logs" etc., may make the work easier to handle in general.
I for one would not mind groupings of finer granularity, best ones providing some more context information than we currently have. I support that idea.
I would also suggest to sort message by audience - so e.g. messages seen by readers-only of the wiki deserve preferred and probably more careful treatment compared to admin-only or oversighters-only messages that only a small fraction of likely computer literate or nerdish folks will ever see, who often are known to read English as well as their mother tongues.
Not going to happen. We have MediaWiki most used, all of core, meta message groups and hundreds of different other extension groups. Yes, it is a lot of work, and it has to get done. Please consider "just translating" instead of talking about (and spending time on) how to make the translation process even more complex in itself or meta-engineering the few thousand messages that exist.
Of course it has to get done. Breaking it into meaningful groups will make getting it done easier.
I'm talking only about MediaWiki, not about the extensions.