I'm working on a new computer language I call Robato. So far, I only have a design document, but I'd like to put it out there and see if anyone would be interested in joining the project. What Stack Exchange site would be the best for doing so? Also, what tag(s) could I use?
That's not quite how Stack Exchange works. You could find a site to talk about issues you have with design. Or even help users of your language.
Stack Exchange by design isn't social - while by some fluke, it has communities, and pulls them together - it's a terrible tool for actually pulling together an external community, and it's been joked it's even anti social by design.
It's also a tool that works very well for small, well scoped problems - and a new language is by definition a very large problem. We do nibbles - you basically need a whole 3 tier cake.
You could in theory ask questions about building a community in the very quiet community building beta and use sites like Stack Overflow or even Code Review to try to polish up and get visibility for your language...
But I think what you're really looking for is GitHub and a little self-promotion through places like Slashdot or Hacker News.