Babelsberg RML : Executable semantics and language testing with RML New programming language designs are often evaluated on concrete implementations. However in order to draw conclusions about the language design from the evaluation of concrete programming languages these implementations need to be verified against the formalism of the design. To that end we also have to ensure that the design actually meets its stated goals. A useful tool for the latter has been to create an executable semantics from a formalism that can execute a test suite of examples. However this mechanism so far did not allow to verify an implementation against the design. Babelsberg is a new design for a family of object constraint languages. Recently we have developed a formal semantics to clarify some issues in the design of those languages. Supplementing this work we report here on how this formalism is turned into an executable operational semantics using the RML system. Furthermore we show how we extended the executable semantics to create a framework that can generate test suites for the concrete Babelsberg implementations that provide traceability from the design to the language. Finally we discuss how these test suites helped us find and correct mistakes in the Babelsberg implementation for JavaScript.