Helping non-English speaking parents to handle HMRC matters

Hi, My parents recently living in UK do not know any English, but they need to submit Self Assessment Tax Return yearly because they receive foreign income. I would like to ask if I, being their Next of Kin, need to apply (register) for the role of "Appointee" or "Intermediary" in HMRC when helping them to handle (complete) any issues about tax return, claim and payment (if needed). If such role is needed, could you please provide me the application details for my further action? On the other hand, what is the differrence between "Appointee" and "Intermediary" in case I am told to apply for either one? Thank you in advance for your expert advice.

Hi KuenHK Lam,
Please have a look at the guidance here:
Self Assessment tax returns
Section 8 covers appointing someone to file and send your tax return.
Thank you.