[ Context: Steve Yegge wrote a blog post about why XEmacs should role over and die. While I agree with his main point, I disagree about his vision of the future for Emacs. ]
I know it is controversial point of view, but see it this way: Emacs is really a lousy OS with only cooperative multitasking. And as a platform, Emacs is a joke compared to the Common Lisp environments. However, as a text editor it is brilliant. And the brilliance doesn't mean it is better editor for a specific kind of text (say programming) than any more specialized editor (say a specialized IDE). The brilliance is that it treats everything as text, and gives a uniform interface to editing (or browsing) the various kind of text. When Emacs is best, it conforms to a matrix similar to the major modes (one for each kind of text) and the minor modes (one for each functionality), where the functionality is adapted to each kind of text.
This view of Emacs, as an application rather than as a platform, is why I say: Please don't add multi-language support to Emacs.. Yes, people like to write applications in their preferred language. But people shouldn't write applications in Emacs. They should extend Emacs. To extend something, you have to read and understand the existing code.
Today, if I encounter a bug or "missing feature" in Emacs, I have to know one, or at most two, languages to fix it. Emacs Lisp in most cases, perhaps C if it is something more fundamental.
In practice, it would likely mean that I'd have the choice between using the Python based LaTeX mode with one set of functionality, or the Perl based LaTeX mode with another set of functionality, because the programmers had different preferences. Yes, there can be multiple packages with similar functionality all written in Emacs Lisp as well. I know, when I maintained AUCTeX, I stole liberally code from Vortex and CMUTeX whenever users suggested features from those packages. All being written in Emacs Lisp and distributed under the GPL made it easy.
Lisp is a good choice for an extension language for a long lived application mostly because of its age and lack of syntax. The lack of syntax means less to learn, and also mean that whatever its popularity elsewhere, it will always have a niche in computer science research.
If you want to make Emacs more OS like, I'd much prefer true multitasking.