Enforcement Rules of the Detention Act Article 41

Defendants with visual, hearing or speech disabilities may use sign language, braille or other proper means for assistance during visits and correspondence.Defendants who are illiterate or cannot write letters for any reason may seek the consent of another defendant or a suitable person to write letters on their behalves; such letters will be sent according to rules after the confirmation and signatures or fingerprints of the defendant.
中文原文 羈押法施行細則第41條
視覺、聽覺或語言障礙被告接見及發信,得使用手語、點字或其他適當輔助方式。
被告不識字或因故不能書寫信件者,得徵得其他被告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.