Note: I was an exchange student in the US, and hosted some US student in return, back in France. So I was able to live the culture shock first hand. 😆
It is true that there is a sharp contrast between the French way of life and other, more easy going countries, like the US.
Politeness is key, especially between strangers. But without knowing any of the rules, a foreigner has a good chance of being rude without even realizing it.
But once you got past that hard outer shell, we can be quite friendly. Of course, there are exceptions, and I personally experienced some pretty rude waiter in Paris… once.