There are PhD programs taught fully in English, you may refer to https://oia.ncku.edu.tw/p/404-1032-230753.php?Lang=en for available English taught programs.
Most undergraduate courses are taught in Chinese, course materials could be Chinese or English depending on the professor's choice. Therefore, students are expected to have adequate level of Chinese.
Besides the listed full English taught programs, most classes in postgraduate programs are also taught in Chinese, with some taught in English, therefore, students should have sufficient Chinese language abilities in order to achieve good academic performance.