The whole point of StackExchange sites is to provide useful information to the whole community. We're not a discussion forum, we're a knowledge base or FAQ place, of sorts, and we want our posts to be understandable by everyone and ongoing discussions about those posts to be public for everyone.
We do allow informality up to a certain extent, but this is taking it too far. My friend Penguin speaks in gamer talk almost everywhere, which I think is okay because it's still understandable. The important thing is that the gamer talk doesn't spread to their questions and answer posts, because that's the most important part of SE sites.
Imagine reading one of our questions, and the answers were in both English and Chinese! It would take a very long time to translate the languages into one you can understand. We want our answers to be easy to read and understand, right off the top! That's why we don't allow languages other than English on our network, because English is the primary language of the internet, and we want our posts to be understandable by the whole community, not just you!
As for comments, when you put your comments in LeetSpeak, it may be cool to you, but it's not cool to others who don't want to have to learn LeetSpeak to find out what you're saying. As I said earlier, we'd like discussion to be public, and when you talk in a foreign language, it's like speaking in cipher code: you're trying to make your discussion private, which goes against the model of StackExchange—to make this a free, open, and public knowledge base that can be read by anyone.
If you want to goof off and chat in leetspeek with fellow members of Arqade, the chat may be a better option for you, however they may be a publicity requirement, so even there it may not be acceptable.