Over the last five years we have built mbeddr, a collection of integrated languages and language extensions built with the Jetbrains MPS language workbench. mbeddr consists of 81 languages, incl. IDE support, 34 of them C extensions. The mbeddr languages use a wide variety of notations – textual, tabular, symbolic and graphical – and the C extensions are modular; new extensions can be added without changing the existing implementation of C. mbeddr’s development has spanned roughly 15 person years so far, and the tool is used in practice and continues to be developed. This makes mbeddr a case study in language engineering of non-trivial size and complexity. In this keynote, I look back at mbeddr’s development to identify what worked well and what are still challenges in language engineering with a modern language workbench. I will address five areas: language modularity, notational freedom and projectional editing, mechanisms for managing complexity, performance and scalability issues and the consequences for the development process. I will close by highlighting a set of issues which need further research and engineering.