You're mistaken about Taiwan, where Japanese is not only spoken by a large portion of the elderly population, but is also the #1 foreign language students choose to learn in schools.

Taiwan still has a huge Japanese speaking population, relative to the time period in which Taiwan was a Japanese colony. They're all just primarily elderly. During the first half of the 20th century Japan had a very effective three-stage policy of language use which ultimately lead to the all-out banning of Chinese languages and aboriginal languages used in public by the end of the 1930s.

The population was so adept at Japanese, in fact, that when the Nationalists moved in in 1945 and banned the use of Japanese, there was a huge amount of push back from the people. The vast majority of aboriginal Taiwanese were out of a job (since they'd learned Japanese and thru knowing Japanese were able to get those jobs in the first place). Japanese had also become the language of the elites, and a large number of the ethnic-Chinese living in Taiwan before the Nationalists came were also upset at this change of policy. People had become very comfortable using the language thanks to the specific stages of the Japanese language policy on the island.

If you want I can go into what that policy was at each stage and why it was so effective, but I'll wait to see if you're interested before I do.

Anyway, this banning of Japanese by the Nationalist government in 1945 was also a significant contributor to the protests against the government in the mid 1940s which ultimately lead to the massacre of civilians in 1947 in what's known as the 228 Incident (for having happened on February 28).

To this day, Taiwan has a large number of people who are still proficient in the language from that time period. Of course these people are now all advanced in age, but it's not at all uncommon to meet someone who is still quite adept in the language. I personally know a number of people who use Japanese as the sole language of communication with their spouse.

In addition to that, Japanese is the most popular foreign language to learn in Taiwan according to statistics from the Ministry of Education. After English, which is compulsory, Japanese is one of the top languages which students choose to study, and bookstores in Taiwan all carry large Japanese language sections. Not just manga but also novels and nonfiction.

Sources:

• Baldauf, Richard B., and Robert B. Kaplan. Language Planning and Policy in Asia. Bristol, UK: Multilingual Matters, 2008.

• Ching Leo T S. Becoming “Japanese”: Colonial Taiwan and the Politics of Identity Formation. University of California Press, 2001

• Ping-Hui, Liao, and David Der-wei. Wang. Taiwan under Japanese Colonial Rule, 1895-1945. New York: Columbia University Press, 2006.

• Manthorpe, Jonathan. Forbidden Nation: A History of Taiwan. New York: Palgrave Macmillan, 2005.

• 大紀元，台修第二外語 印、越語潛力大，2011/08/30

• 国际在线，台湾高中设七种第二外语选修课 日语课最热门，2007/01/03

Japanese was the language of schools in Korea while Japan controlled it from 1910 to 1945. Pretty much everyone growing up at that time would have learned Japanese.

I speak Japanese fluently and visited Burma, and found that a significant number of elderly Burmese people I met, who had been students during World War II, could speak Japanese.

A significant percentage of Filipinos learned Japanese during occupation in World War II as well.

However, once Japanese control was over, most countries that had been controlled by Japan had no interest in maintaining ties to Japan. They had been forced to learn Japanese and when they were free, didn't speak it or use it much. Japan had not been in control of those countries long enough to prevent people from learning and speaking their indigenous language, so they naturally went back to using it. Japanese wasn't particularly useful to them once the war was over, either, not being an important language for international relations or trade. However, English continued to be useful to former British colonies since English is a major language for world diplomacy and business.