What's the weather like in western Europe (France, Italy, Switzerland) in mid-May (around 10-20)? What clothes do you need to bring? thank you

No, just wear ordinary spring and autumn clothes at this time. Of course, people in China are not as healthy as others. You can wear one more, but down jackets are generally not needed. A woolen coat with autumn clothes is definitely enough. At that time, the weather was cold at night, but the temperature was ok during the day.