European alternatives to Slack

Stackfield is a project management and collaboration platform from Germany, combining all the features teams need for effective collaboration: team chat, task and project management, whiteboards, video conferencing, collaborative document editing, and more.

Element is an open-source team communication app from the United Kingdom, built on the open Matrix protocol. Rather than routing every conversation through a single company's servers, Element connects to independent Matrix homeservers, so organizations can run their own server or rely on the public matrix.org homeserver that Element operates on behalf of the nonprofit Matrix.org Foundation. All conversations are end-to-end encrypted, and Element requires verified devices before they can send or receive encrypted messages. The apps are available for web, desktop, iOS, and Android, and existing Slack, Microsoft Teams, or IRC channels can be bridged into Matrix rooms. Element is used by companies, public administrations, and individuals who want a team communication services tool that does not depend on a single vendor's infrastructure.

Threema Work is a business messaging service from Switzerland, built for companies, public authorities, and other organizations in the team communication services category that need confidential internal and external communication. It offers end-to-end encrypted text and voice messages, group chats, video calls, and file sharing, together with centralized user and license management through a web-based management cockpit. Client apps are available for iOS, Android, Windows, macOS, and Linux, and a separately offered self-hosted variant called Threema OnPrem exists for organizations that need full infrastructure control. Threema Work is a distinct subscription product from Threema Private, the same company's consumer messenger, and the two can be used independently of each other.

Infomaniak kChat is a unified communication platform, where you can communicate live with your teams and organize projects in a secure and instant way, from the Swiss cloud provider Infomaniak.

Nextcloud Talk is a privacy-friendly open-source video conferencing and team collaboration solution. Nextcloud Talk enables secure collaboration for hybrid teams and improves productivity and connectivity from anywhere. Without lock-in effects and dependency on proprietary providers, its flexible open-source architecture enables customization to the specific needs of companies and seamless integration into existing systems. Nextcloud Talk is part of the Nextcloud Hub a self-hostable platform for companies and private customers.

Fleep is a team communication service from Estonia that offers many integrations and Apps for iOS, Android, Windows, macOS and Linux. They don't offer audio and video calls directly in their application, but they do offer integration with an external tool, so it's easy to start a call from the chat window.

Mattermost is an open-source primary self-hosted team communication service aimed at developers but suitable for all kinds of teams.

Stackhero.io is a France-based managed cloud service provider that offers a managed instance of Mattermost Cloud.

Wire is an team communication service from Switzerland with Apps for iOS, Android, Windows, macOS and Linux. The communication is end-to-end encrypted and the apps are open source.

Nozbe is a team communication service from Poland. They incorporate many features, like instant messaging, threads and kanban, within their Apps available for most platforms. In contrast to other team communication tools, they develop for mobile first, but desktop applications are also available.

ginlo Business is a team communication service from ginlo. ginlo is a company from Germany that also offers ginlo Private, a instant messager for private customers.

Private Discuss is a team communication service from France with a stron focus on audio/video conferences. They also offer instant messaging and apps for all major platforms.