You are underestimating German federalism - there are several competing providers for each specialized software used by German municipalities, often with huge customisations. There is no way you could mandate a specific product. What can be done from above, in some cases, is specifying requirements (for eg data storage, privacy laws, code auditability...) and APIs (like APIs to federal agencies) and let the customers and providers create solutions. It might be messier, but in the end you are not dependent on a single product.
