Programming language frameworks allow us to generate language tools (e.g., interpreters) just from a formal description of the syntax and semantics of a programming language. As these frameworks tend to be quite complex, an issue arises whether we can trust the generated tools. To address this issue, we introduce a practical formal programming language framework called Minuska, which always generates a provably correct interpreter given a valid language definition. This is achieved by (1) defining a language MinusLang for expressing programming language definitions and giving it formal semantics and (2) using the Coq proof assistant to implement an interpreter parametric in a MinusLang definition and to prove it correct. Minuska provides strong correctness guarantees and can support non-trivial languages while performing well.