Note: This text is an English translation of the article 誰愛說國語？（一） by Richter. Both the text and image are his original works.
Translator's Note: "Guo Yu" in Mandarin literally translates to "Language of the state".
The "Guo Yu" in 1930s Taiwan is, of course, not Mandarin as it is today, but rather Japanese. The exact same phrase, conveyed a different underlying meaning. Obviously the term "Guo Yu" is not a specific linguistic term, but a political expression.
Seventy years before present time, who spoke "Guo Yu"?
We gain insight of the popularity of Japanese from a survey conducted in 1930. Interestingly, the east and the so-called "barbarian areas" (areas occupied by aborigines) had a higher level of penetrance relative to the rest of the island. This is explained by the fact that Japanese authorities of the time had put a great deal of effort into "assimilating" the aboriginals with quite some results. In fact, to this day, Japanese is still a common language among the elders of aborigines. On one occasion, I actually had to rely on a Taiwanese-Japanese translator to communicate with the aboriginal elders.
Excluding the east and "barbarian areas", three trends of distribution of Japanese were observed. Not surprisingly, higher prevalence was found in urban than rural areas. Over 1/4 of the local population in Taipei city, Taichung city, and Tainan city spoke Japanese. Hakka acceptance of "Guo Yu" seemed wider than Holo people both in the north or and in the south. Other than this, using the Jhuoshuei river as the border, the north part of Taiwan spoke "Guo yu" more readily than the south.
Intuitively, we expect the portion of Japanese in the population, the population density(reflecting the urban-rural differences), and the composition of the local population (Holo/Haaka/Aborigine) fully explains the prevalence of Japanese. However, as regular readers might have expected, language is the most typical collective behavior. Unless one lives in a totally isolated environment, the language we choose to use is often decided by the language commonly used around us. The more people use a particular language, the more likely we choose to use this language. The popularity of a language is obviously a type of "diffusion".
Unfortunately as a student, I don't have the time to deeply research this topic. Nevertheless, I am going to point out the distribution pattern of "Guo Yu" today is identical to the one 70 years ago, although they are two completely different languages.