We present a type system over language definitions that classifies parts of the operational semantics of a language in input, and models a common language design organization. The resulting typing discipline guarantees that the language at hand is automatically type sound. Thanks to the use of types to model language design, our type checker has a high-level view on the language being analyzed and can report messages using the same jargon of language designers. We have implemented our type system in the LANG-N-CHECK tool, and we have applied it to derive the type soundness of several functional languages, including those with recursive types, polymorphism, exceptions, lists, sums, and several common types and operators.