First of all I suggest familiarizing yourself with the reasons as to why Lua was "ported" to MediaWiki (I use ported loosely because the implementation is more of a bridge between MediaWiki and the Lua interpreter).
Secondly, you need to ask yourself this question, "Why would the language I use to generate content matter to a search engine?". If you come up with anything more than it doesn't you should take a refresher course on what SEO is and how it works.
To a search engine what generates the content does not matter, it's what the content is that matters. A search engine could not care less if you used ALGOL or Turbo PASCAL to create the page just so long as it can output a readable page.
Now, I am by no means saying that you shouldn't do these things, but I am questioning why you would.
As for a diverse programming language and SEO, no, no it does not effect SEO at all, though it does considerably increase the complexity of any given system.
Lemme know what you think of this. I'd love to hear back from you.
As to your question of "Why would the language I use to generate content matter to a search engine?", the answer is "I don't know". I tend to avoid AI research in general, which includes search engine crawlers. I do not know if programming language diversity would affect those crawlers, even though I sort-of understand the pagerank algorithm. That's why I asked.
In summary, the reason I'm planning to make these languages available is so the developers have more and better tools. I know that's a questionable goal here, but so is roughly 80% of the stuff I code, so that's just fitting a theme.
If language diversity doesn't introduce any problems, then today is going to be fun.
As for LISP, you are correct, they were made for two very different things. This begs the question, why would you want to use it on the web? No modern browser has a built in LISP interpreter so LISP cannot be used client side. At best it can be implemented server side but again why? We already have Lua and it suits the way wikis work very well.
I'm aware they can already do that. The solution I'm making is simply more direct.
We can get it working client side. That's off-the-shelf. To answer your question of "would you want to use it on the web?", the answer is "Who knows? Let's make this available and see what people make!". I actually don't even like LISP. I'm just making usable for people who do.
Yes, we have Lua. And with what I'm making, we'll have a few other languages. Yes, Lua works well. This way, some other things will work too, probably poorly.