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The important metric is not "how many people live in English-speaking countries". The important metric is "how many people speak English". There are several times more non-native English speakers than native English speakers.

Mandarin doesn't seem to be very popular beyond native speakers, and the Chinese population is shrinking. I wouldn't bet on it in a race for the next Lingua Franca. Though I can imagine a Firefly-like future with Mandarin words mixed in with English.