The official language of Taiwan is Mandarin Chinese; however, regional languages include: Taiwanese and Hakka.
Taiwanese is the local dialect of Taiwan, but the majority of locals can speak both Taiwanese and Mandarin.
The choice of using Mandarin or Taiwanese usually depends on the context of the situation. For instance, a more formal environment would require the use of Mandarin.
Hakka is the other regional language in Taiwan and is mainly spoken by the native Hakka people. This language was primarily used in isolated regions throughout Asia, so a variety of dialects exist.