Drink water. Stay in shade. Wear hats. Wear long sleeves, sun on the skin is hotter than not. Drink water. It's not a secret that air conditioning is popular in the US but if you look at a map for 20 seconds you'll realize most of Europe is north of "The American South" so acclimation plays a key part in resistance to weather. If you are doing physical activity like bike riding or running then take more breaks. It's not rocket appliances.