Mark 9:12 is translated in the 8th Century translation of the gospels into Arabic that is found in Vatican Arabic MS13 in a remarkably clear and communicative manner. It says:
“Elijah comes first to prepare all things (and perfect them and complete them), so how have the scribes understood this about the coming of Elijah but they do not understand what is written about the Son of Man that he will suffer much and be rejected?” (قال لهم الياسين ياتي اولا ليعد الاشيا كلها ويكملها ويتمها فكيف فهم الكتبه هذا في مجى اليا ولم يفهموا ما كتب على ابن البشر انه يصلى كثيرا ويرذل)
Source: Andrew Persson