An XML language can be created to express Bookstores.
I think, that won't be a language. It'll be a schema of bookstores data.
For example, XSLT or XSD are languages created using XML syntax.
How big is the set of possible XML languages?
I think, theoretically this can be countably infinite.
Is the set of possible XML languages a superset of the set of possible JSON languages?
My feeling is, yes. Combining XML with XML namespaces and XML Schema, would allow much more languages to be defined using XML syntax, than JSON alone (but I've heard, that JSON has a schema language too).
