In the 19th century, communication was extremely inconvenient, so the citizens of the Far East Empire had a very basic understanding of global affairs. Many people couldn't differentiate between England, France, Germany, and so on.
Mostly, they just went by hearsay about the world's powerful countries, categorizing many nations as foreigners. The Western world naturally was Westerners, while the East really just referred to Japan, naturally called Easterners. Of course, due to historical reasons, Japan within the Far East Empire had its own nicknames, like Japan or Fusang.
The Far Eastern workers employed by East Africa naturally couldn't distinguish all these countries, so they followed the East African troops, calling Japan "little devils" and such.
The term "devil" wasn't exclusively derogatory towards Japan; invaders of the Far East Empire earned this label. Western devils, British devils, French devils, though Japan uniquely earned the title of "little devils."
