Pokémon world in relation to the real world/Core regions/Sinnoh

Sinnoh has been said to be an island like Hoenn, rather than a peninsula. It is based on the northernmost major island of Japan,. Part of (Japanese: 国後島 Kunashiri Island, Russian: Кунашир Kunashir), which is in dispute between  and Japan, is also included.

Battle Zone
The Battle Zone is based on (Japanese: 樺太 Karafuto, Russian: Сахалин Sakhalin), a Russian island that used to be claimed by Japan (some parts of Sakhalin are still in dispute, but none of them are the basis for the Battle Zone).