To clear some misconceptions:
1. using Schemas will not make documents valid. Compliance to Schema will.
2. using Schemas will not make XML language independent. XML *is* a language, how can a language be "language independent"? If by XML you mean "specification of its structure", be it DTD or Schema or something else, than preference of one language (XML) over another (EBNF) to express it will not make such a specification language independent either.
Accidentally, "XML Certfication" forum
is discussing this subject today I posted a quote from David Mertz's "Comparing W3C XML Schemas and Document Type Definitions (DTDs)" there.