Assembly, arguably the most known demo party, was famously Finnish. Future Crew (who made Second Reality) were (are?) Finnish as well. Lots of demo scene in Germany, also.

About evenly spread both sides of the Iron curtain, I’d say. Lots of demoscene in eg Germany, Scandinavia too. Netherlands, UK and France also have sizeable (though smaller) scenes.

And to lesser extent Iberian Penisula, where sceners like shader toy creators come from.