First up thanks for making the discussion public!
First up is the proposal to expand into other languages. I see no reason NOT to do so, but the problem is in the HOW and WHAT TYPES. Putting another language on a footing with PHP can quite easily stab you in the back - general forums in my view lack a certain community feel and are easily led based on who holds the knowledge - not the users. Sitepoint is a classic example - I dislike it for many reasons but the two main ones are the pedantic posts and the ads which play havoc when you're using dial up.
The one thing I don't understand however is the approach... Has anyone formalised any planning?
You could expand in two directions - within PHP, or outside it. Now both could work, the question is which is more effective (mixing is not criminal either

) and which can leverage off the strengths of what already exists?
If you take one perspective expanding beyond PHP into other languages may have several effects.
First, you'd be competing with a pre-existing solution.
Second, you'd be ignoring the forum focus - maybe.
Third, it could wind up diluting the PHP signal.
Extra languages may pull regular users between both camps, dividing effort, and lowering the current level of activity and help. So it can come down to a question of the user base you could attract - the more users, the more participants helping.
It could however work - but then the question becomes one of marketing. I know the M word is evil and all, but without gaining new users at a reasonable pace the exercise could be pointless.
So which languages? Although I know C/C++ would be popular to do (many are familiar with it), I can't see a new forum errupting. There are C/C++ forums already established that most people use regularly. Same goes for Perl, plus Perl is not growing these days. The same could be said of Java, etc. In my view some (or all) are more suitable for single forum sections. Not a whole new forum.
One could suggest picking something that's expanding, that has a lot of potential, and a lot of confused users just entering the learning phase, e.g Ruby, Python, etc. I could see DN making an impact there a lot more effectively than in languages where the user base is established and growth is slight or even in decline. Entrenched user bases are almost impossible to crack.
The other side from expanding the languages is to expand the PHP network. PHP is pretty lax on that front. One of the symptoms is the PHP Google Bug - if you want a PHP related resource, you search Google. Why? Because there is no other obvious list of resources... Now I know they exist - some of them. But in my experience many exist in isolation, or are simply ignoring user needs in favour of their own needs, some are downright horrible. Others are very high quality - but not something a new user would easily find. The majority operate via word of mouth alone almost... I still discover PHP gems by following the blog community that are never mentioned anywhere else - not even a small news bite.
As to which path would reap the most benefit, I'd have to say the network. I'm not entirely certain even these forums are as successful as they could be with the right conditions (and not all of those are reliant on a network, e.g. the Google search for "php forum" returns Devnetwork without a decent description of the forum - a useability flaw in my view. Minor but this is seen by how many PHP users a day?).
Of course all those Network components need to exist to be effective - so maybe that can be discussed separately.
Back on the proposal for expanding the language set...
I think it could make sense, but not in all cases suggested. IMO I think Perl and C/C++ would be a mistake. Both have an entrenched user base, so both have high barriers of entry for yet another forum. Picking something less fixed, with a growing user base might prove a better choice. Ruby for example, although with its own forums, is growing fast. A simple "ruby forum" search indicates lots of room for another Ruby forum. Sitepoint is currently in there. The rest listed are largely seeing minute activity, or are lists of other resources. None come close to what Devnetwork resembles for PHP.
It could also be suggested to stay within sight of the current user base's experience. PHP and Ruby though competing languages in some aspects, could be complementary for a forum network. Plus as Ruby grows, so will the user base, the accumulation of knowledge, etc. I see no reason why DN couldn't fill that opening...
Of course Ruby here is an example - the same could be true of Python, or other languages - the point is to choose with care, and keep the focus narrow. Dumping a dozen new languages into new forums probably isn't the plan anyway...
To end this ramble everyone gives up on this post!
I would prefer to see the Network developed, than an arbitrary set of new forums suddenly erupt and bury PHP in the noise of many. Trying to do too much would probably have the same effects as it sometimes may in a software application - it stops doing one thing really well, and only does it okay - sometimes it does more harm than good. I wouldn't categorically reject a language expansion - but it should make sense, and have some connection to the PHP user base who are generally developing web applications. A jump from PHP to C/C++ from that view makes little sense. Then again as someone pointed out - there is room in PHP for a forum on PECL.