Japan is a country in the Real World. It is an island chain in the Pacific Ocean off the coast of Asia, and has a rich and interesting contemporary and folk culture.
Many fangirls and fanboys consider Japan to be the most awesome place in the world simply because it is where a lot of their favorite fandoms come from. They often blatantly ignore the full culture of the country, reducing it to a kind of "animeland" based only on what they as fangirls have squeed over. In reality, it has its problems and pitfalls, the same as any other country. Many fangirls and fanboys in fact do not know the first thing about the country—if you asked many of these fangirls, they would not be able to tell you the current prime minister of the country they idolize so much.
This results in many short-skirted, crazy-haired Fangirl Japanese-speaking Sues who fail to use honorifics properly. Thus, it is an important place for the PPC to keep an eye on. Other than simply Sues, there are many badficcers who believe anything is cooler with a Japanese name and so will name everything and everybody in Japanese, regardless of whether it would be appropriate or not. Ninjas are also popular, as is Japanese weaponry—expect them to show up in badfic spanning continua from Pirates of the Caribbean to The Lord of the Rings to Tron.