What language is the most commonly spoken language in Taiwan?

From my understanding, Mandarin(Putonghua) is the only official language in Taiwan.
But which language do Taiwanese mostly speak at home with family and friends in everyday life?
Is it Mandarin(Putonghua), Min Nan/Hokkien(Taiwanhua) or Hakka?

Also, is it true that most older generations in Taiwan speak fluent Japanese?

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Filed under: Mandarin Written and Spoken