&gt; In contrast, a schema language allows extensions to be described once, in one language.<p>No, it doesn&#x27;t. A schema language allows that to be done for the syntax, but requires the semantics to be implemented for each language. Schema languages often <i>include</i>, as part of their specifications, core types which must be supported; when an application restricts itself to these core types and types whose only important semantics are derived from them (e.g., restricted subsets of core types in most cases), then the fact that every full implementation that supports the core types will already have this work done for every language means no additional work is necessary. But that&#x27;s not a product of a schema language preventing types to be implemented for all host languages, that&#x27;s a result of the fact that a predefined set of core types accompanying the schema language means that all implementations are required to have already done the work of implementing the core types for all languages.