I've worked in companies with a presence in various European countries over my career. Whether or not everyone takes Summer leave at the same time very much depends on the company, and the country. I specifically remember working with a Finnish contractor firm who planned to have no billable time available at all in August, from anyone. But our offices in France and elsewhere never fully shut down in August, they were just very lightly staffed. Everyone took some multi-week summer holiday, just not the whole place at once.

It's not just summer leave, either. There are people all over the world having kids and going out on maternity (or even paternity!) leave for months at a time. When my wife and I had our kids in the US, I didn't get any extra paternity leave, and just used saved-up PTO. I particularly remember that my wife had to stay in the hospital for a bit after my first kid was born, so the two weeks I had saved up flew by in a flash. I recall my boss strongly encouraging me to dial in to a conference call on that last PTO day, and when I did his boss lashed into me for taking so much time off. I started sending out resumes shortly after.

On the other hand, when the Europeans I worked with later got their summer or parental leave, their Project Managers just dealt with it, and if it meant their schedules had to slip, they slipped, no temper tantrums required. And I think that is the key difference. American bosses and PMs are much more likely to get away with assigning blame for schedule slips downward, perhaps because not as many people are unionized.