A prime decision of engineering domain-specific languages (DSLs) is whether to implement these shaped as external DSLs or internal DSLs. Agile language engineering benefits from easily switching between shapes to provide rapidly developed prototypes to the customer before settling on a specific syntax, but this switching rarely is feasible due to the effort of re-implementing language tooling for both shapes. Current research in software language engineering focuses either on internal DSLs or external DSLs, whereas deriving internal DSLs and external DSLs from a common syntax description can support agile language engineering. Therefore, we conceived a method to automatically derive an easily customizable, internal DSL from a grammar. The internal DSL operates on the same abstract syntax representation as the external DSL, as both are derived from the same grammar. This supports reusing tooling (such as model checkers or code generators) between both shapes. We realized our concept with the MontiCore language workbench and Groovy as host language for internal DSLs. This concept is applicable to a wide range of grammar-based language definition formalisms and can facilitate agile language engineering.