German speaking part of EU.
Given the fact, it is most likely that only German speaking part will be the central core of FKK set up, then it may spread to surrounding border area of other nations like YY in Netherlands, which I presume got inspired by German FKK which is just across the border.
I always found German speaking part and German girls or German born raised girls more kinky than others in FKK, why this is so if true, I do not know. I find Germany and German speaking part more sexually liberal than other nations in general.
America being quite religious or even very religious in many states with so many feminist and human right movement going on, I highly doubt that such a extreme liberal set up as German FKK would be so openly legal in USA. South America can be more chance for similar set up as therma in Brazil spreading to surrounding nations.
Latin girls are beautiful and good in bed, so that would be very nice movement to see. But we occasionally get girls in German FKK from south America.