Singapore is a cosmopolitan port city, there are several Chinese languages spoken there, and Mandarin was not the dominant one until recently. There are also many other languages spoken in Singapore, and from what I understand English is the primary language used for official business. Taiwan was not natively Mandarin speaking but speaks it now because it was taken over (from the Japanese) by the fleeing Mandarin-speaking KMT after they were beaten militarily by the Communists during the Chinese Civil War. Both Singapore and Taiwan were ruled for decades by authoritarian governments. I’m not sure about Singapore but in Taiwan other Chinese languages were forcefully suppressed.
Plenty of other parts of the world manage to communicate across regional/national borders without restricting people’s ability to produce/distribute local media in their native languages.
There are many countries where students learn several languages in school (including their native regional language and a national language) from an early age.
(Disclaimer again: I’m not an expert in the history, politics, or comparative linguistics of China. I recommend Wikipedia as a better first summary, if you are curious to learn about these subjects.)