Add a demo to show live swapping of languages

[rachel-fenichel]

The Plane demo currently uses soy templates and closure to do language swapping. That's not especially interesting anymore, but we could adapt it to do the same thing but with our preferred way of swapping languages. We could also have live theme swapping in the same demo.

update: the plane demo has been deleted. it would still be useful to have a demo of live swapping languages/themes/etc.