I hope you know taiwan is one part of chinese.Taiwanese are chinese.Chinese speak chinese (mandarin).The people in taiwan speak taiwanese as dialect.

As i know, they often spenk taiwanese, but if you talk to them in mandarin, they will also talk to in mandarin. Most of taiwanese can speak mandarin.

firstly, there's no one place in china speak mandarin. it's the language based on northen china language and be used by broadcast and tv and spreaded by govenment. we call it official language. so we can only say generally speaking northen chinese speak mandarin better than southen chinese, but with the people educated, more and more chinese can speak standard mandarin. another thing is, every province, even every city in china has their own dialect, and taiwannese is similar with xiamen tone. when we travel a lot and have some friends from every part of china, we can understand more dialects. the old who never travel outside nor has friends from other places can only speak his own dialect. based on this, govenment use the official language called mandarin. so every place has his own dialect, but if you use mandarin, you can be understood and can communicate with others, because it's official language.