Well, I would say that XSD is indeed a "formal computer language", but not a "programming language"; I don't think you can describe something as a programming language unless it is Turing-complete.

I don't know of any universally accepted categorisation scheme for formal computer languages, and without such a scheme you can't say where a particular language fits; but it's certainly reasonable to describe XSD as a constraint specification language or as a data definition language (if indeed those two categories are distinct).

Michael Kay
Saxonica