The proliferation of independently-developed and constantly-evolving domain-specific languages (DSLs) in many domains raises new challenges for the software language engineering community. Instead of starting the definition of new DSLs from scratch, language designers would benefit from the reuse of previously defined DSLs. While the support for engineering isolated DSLs is getting more and more mature , there is still little support in language workbenches for importing, assembling, and customizing legacy languages to form new ones. Melange is a new language workbench where new DSLs are built by assembling pieces of syntax and semantics. These pieces can be imported and subsequently extended , restricted, or customized to fit specific requirements. The demonstration will introduce the audience to the main features of Melange through the definition of an executable DSL for the design and execution of Internet of Things systems. Specifically, we will show how such a language can be obtained from the assembly of other popular languages while maintaining the compatibility with their tools and transformations .