Since Discord has decided to set itself on fire you might be looking for an alternative. Fluxer is open source (AGPL), selfhostable and based in Sweden. But maybe wait a while before you try to install the server yourself because the dev writes on github:
I know it’s hard to resist, but please wait a little longer before you dive deep into the current codebase or try to set up self-hosting. I’m aware the current stack isn’t very lightweight. In the next update, self-hosting should be straightforward, with a small set of services: Fluxer Server (TypeScript) using SQLite for persistence, Gateway (Erlang), and optionally LiveKit for voice and video.
The developer hosted instance has a Nitro-like premium tier for their service named Plutonium, if you selfhost you get all the premium features for free.
Fluxer is not federated (yet), but according to the website, federation is planned in the near(-ish) future. Furthermore the service is a bit spotty at the moment as they are fighting with an unusually big influx of new users.
Fluxer really looks great. Might be the greatest alternative to discord, made in EU. But fluxer is tired these days. Too much traffic…
Why not Stoat? Main dev is in the UK so unfortunately not in the EU, but it’s been around a lot longer than most of these alternatives, and seems to be the main one i hear about
It looks very nice, but unfortunately it’ll likely not really reach mass-appeal just due to no proper mobile app (yet).
Personally I’ll definitely look into moving over with the communities I really enjoy using - but the masses will not move in my experience. You can see with how reddit still exists.