The Enforcement Rules of the Prison Act Article 47

Prisoners with visual, hearing or speech disabilities may use sign language, braille or other proper assisting methods during visits and correspondence.Prisoners who are illiterate or who cannot write letters due to any reason may seek the consent of and ask other prisoners or proper persons to write letters on their behalves; such letters shall be sent in accordance with the regulations after the confirmation and affixation of signatures or fingerprints by the prisoners.
中文原文 監獄行刑法施行細則第47條
視覺、聽覺或語言障礙受刑人接見及發信,得使用手語、點字或其他適當輔助方式。
受刑人不識字或因故不能書寫信件者,得徵得其他受刑人或適當之人同意後代為書寫,經本人確認並簽名或按捺指印後依規定發送之。

English translation for reference only; the Chinese text prevails. Not legal advice.