Japan is (and was) a pretty ethnically homogeneous place; the largest indigenous minorities are the Ryukyuans and the Ainu. The Ryukyuan languages aren't all th

Japan is (and was) a pretty ethnically homogeneous place; the largest indigenous minorities are the Ryukyuans and the Ainu. The Ryukyuan languages aren't all that different from Japanese in the first place, and the Ainu were subjected to forced assimilation that has made many ethnic Ainu unable to speak the language, and while I'm not sure of the speaker count in the 1940s, I doubt it was that high. While their imperial possessions at the time would have included lots of other ethnicities, incorporating Korean or Chinese code talkers wouldn't have been that helpful (not that hard to find people who speak those languages), and the size of their Pacific Island groups would have likely struggled to produce enough adequate code talkers. Although the institutional racism of the country was probably enough to forestall any plans in the first place. It's also worth pointing out why the use of Navajo code talkers was so successful. Part of that is that it's from a language family that was poorly known by linguists at the time. But even within that language family, Navajo is pretty different (compare how different English is from other Germanic languages, or indeed most Indo-European languages). It's also a fiendishly complex language; like, there's no regular verbs--that would make attempts to crack it via standard cryptanalytic techniques rather more difficult. (Also worth pointing out, it was a code within Navajo so that even when the Japanese pressed a Navajo POW into translating the messages, they couldn't crack it).