Coming up in my garden right next to some forest (primarily oak mixed with beech, birch, maple) in southwest Sweden. Last year i dug the area out and removed (most) roots. Not much came up but this little sprout. I suppose it could be roots reaching in from the adjacent forest.
Have you got any native type of cane in Sweden? This really looks like something very close to cane or bamboo. Search for images of their sprouts online, compare with any other tree. Not even palm trees look like this when sprouting.
Looking forward to your update on this once you know what it is