I believe I just answered you at Kickstarter too, but I don't want to leave this unanswered. Thanks for reaching out on it.
I think we can find some fun ways to introduce international holidays, through additional content as we go. For the basic printed game it really comes down to this - we are an American company with the vast majority of our games reaching American audiences. This game will be licensed to other countries and customized to them, and if we were able to make an international edition that can actually get into international markets, we would update that. I also like your national day approach a lot, but I think that could come as additional content down the line.
But for PNP specifically, it's VERY EASY to make alternate versions of that holiday card. We should start asking for regional holidays. Might be a good Kickstarter update post.