Element

Element is an open source app for team communication built on the open Matrix protocol. Conversations are end to end encrypted, and because Matrix is decentralised, Element can talk to other Matrix servers and to people using entirely different Matrix clients.

Element is listed here as self-hosted and open source, so it is not necessarily European. It becomes a European alternative through where you run it: the code is open and you choose the server, which can sit on European infrastructure. It suits teams and communities that want federated chat under their own control.