Several years ago I was in Paris for the transition period between the end of August and the start of September. The contrast between the two was interesting: in late August the predominant language (one heard on the streets) was ENGLISH, a day or two later, it was FRENCH.
Over the years, the vacation period has been staggered; back in 1964, when I first went to France, whole areas of Paris were "ghost towns" in Aug.; now things are better.