#680 - Russian Frontier

The northeast part of the Sinnoh region is based on the Sakhalin Island, which belongs to Russia. However, from 1855 to 1875 and from 1905 to 1945, Japan and Russia agreed to give Japan the southern part of the island, which is the one we see in Sinnoh.

SOURCE 1: https://en.wikipedia.org/wiki/Sakhalin#Division

SOURCE 2: https://bulbapedia.bulbagarden.net/wiki/Sinnoh#Trivia