Maybe not “all” demoparties around the world but for the vast majority of them, yes absolutely! The moment you enter the party hall, English will be the primary language for communication between sceners / visitors and the organizing team. Competition announcements, beamslides on the bigscreen, the intranet with its compo entry submission system and videostreams will all be in English most of the time.
That’s not to say that people won’t speak other local languages as well of course. You may encounter this more often at small demoparties in the far north, east, south or western regions of Europe, for instance when friends or groups are having conversations with each other. But I yet have to find anyone who does not speak English at a demoparty, it really is no problem 99% of the time 