What is this plant with distinctive leaves?
Seedlings are often tricky to determine, but I think this may be the common weed Chenopodium album (from the family Amaranthaceae). This is common across Europe, and the leaf shape, growth pattern and the silverly underside seen on the top leaf in your picture fits well.

Here are pictures of a seeding and a larger plant:


(from flickr.com)


(from https://plants.ces.ncsu.edu)