What should I pack? Hot or cold. Most of Sept in Germany, Paris, Switzerland and Italy
What parts of Germany, Switzerland and Italy? And when?
Both -- using light weight layered garments it is easy to do. Except for Italy the others could be chilly especially in the evening or early morning.
I recommend wearing all of the clothes.
Given altitude in the Alps, temps could range from freezing and snow in the mountains to 100 Deg F closer to the sea in Italy.
I pack for anything from 32 degrees Fahrenheit to 80 degrees. I will be uncomfortable at either end of the spectrum but that's about the best I can do and stick to one 22 inch bag. I have lightweight layerable pieces with some fleece, a pair of tights, and a good rain jacket, as well as Gortex footwear.
Not sure what weather brings, but we have had a cool and wet summer in Germany this year. In the 60s F today in Northern Bavaria. I'd pack for cooler and rainy weather here. Bring or purchase a good collapsible sport umbrella.