I would suggest the following corrections:
"I am writing to ask you for an extension for the university enrollment deadline on 8th October. As my visa appointment will be on 29th August, I will not be able to enroll in time. I was wondering if it would be possible for you to send me an extension letter via email. I was also supposed to receive a letter indicating that the LENC program is in English, and that my current TOEFL score, 517, is acceptable by the university, but I have not received it as of yet. I would appreciate it if you could send it before 29th August."