Governments in Germany, France, Poland, the Netherlands, Luxembourg and Belgium have begun rolling out their own messaging apps to prevent employees from using widely used encrypted messaging apps to share sensitive information and to shift to local alternatives that can be monitored. NATO also has its own messaging app, and the European Commission plans to launch one by the end of the year.
The Swiss government uses Threema, which can be self-hosted. They can’t monitor the contents though.
Threema is great, I’m using it privately a lot. Several German authorities have set up and published variants of Element X, using the Matrix protocol in the backend.