It's mainly influenced by the Japanese school system, but also uses some aspects of other school systems. I'm assuming they won't be using certain Japanese breaks like Golden Week, but we aren't far enough in the game to know whether they'll put those breaks under different names.
The Japanese school year begins in April. The first term runs to around July 20, when summer vacation begins. Kids return to school in early September for the second term, which lasts until about December 25. The final term begins in early January and continues to late March.
While this is speculation at best, I hope it helps! When the next update comes out, be sure to ask this in the next AMA.
Pokemon region are the representation of multiple countries in our world, so it will make sense there will be multiple celebrations from Japan, USA and France. But we are in Kobukan, and by the name we can easily think it is asian based, and mostly japanese, so the majority of things, celebrations, themes, and everything cultural can be from the 3 countries but in majority japanese for my opinion.