Legal requirements are here.
Article 4
A foreigner in the category referred to in the preceding paragraph, who is employed as a foreign language teacher at a foreign language center at a public or a registered private university or tertiary college, shall have a degree recognized by the competent authority, from a domestic or foreign university or independent college, and any language course taught by that person shall be a course on an official language of the nation that issued the passport of that foreign teacher.
Act of Governing the Appointment of Educators
https://law.moj.gov.tw/ENG/LawClass/LawAll.aspx?pcode=H0150017