The South Pacific Mandate was the result of Germany losing World War I and Japan, after taking several German possessions in 1914, becoming the recognized new owners by order of the League of Nations in 1919.
The Japanese were very interested in gaining these lands, especially for an increased empire. Also the Japanese Navy took the opportunity to field explorations under the assumption that they would find roots to Japanese ancestors. They sent archaeologists and other experts to explore the islands and would make a good addition to possible NPCs the characters meet.
Since this Japanese territory is the province of Princess Koji, I'll have to do a write-up on her at some point. After all, if she's a good enough frenemy for Jake Cutter, she's good enough for the PCs.