I don't have firsthand knowledge, and I can't guarantee anything, but this is something I found on the Ministry of Education website:
International applicants above the age of 18, who possess a high school diploma or above, have shown excellent academic performance, and are of good moral character are eligible. Candidates with one of the following qualifications may not apply:
a.Is an overseas Chinese student or a national of the R.O.C.
b.Is currently registered as a student at a Mandarin Language Center or has sought a degree from a university or college in Taiwan.
c.Has already received a HES or a Taiwan Scholarship, in the past.
d.Is an exchange student to Taiwan, during the scholarship period.
e.Is currently receiving financial benefits from the Taiwan government or another educational institution.
This is from another page on the Ministry of Education website:
08 Huayu Enrichment Scholarship
An applicant must meet all of the following criteria:
A. Be of 18 years of age or older, a high school graduate or above, have an excellent academic performance in his/her most recent formal educational study experience and be of good moral character.
B. Is not an ROC (Taiwan) national.
C. Does not have the status of being an overseas compatriot student.
D. Has never been a student at a Mandarin training center, nor a degree-seeking student at any university or college in Taiwan.
E. Is not an exchange student in accordance with an agreement of cooperation between an international university/college and an educational institute in Taiwan, at the time of receiving a HES.
F. Has never received an award either under this scholarship program nor the Taiwan Scholarship Program.
G. Is not a recipient of any other scholarship or subsidy offered by the Taiwan government or any other educational institutions in Taiwan.