Is it known from any trusted source what the Engineer was saying here? David was clearly saying "I told him he want to live forever".
At this time, nothing has been published regarding the full conversation. However, one website contacted the consultant who created the language spoken by the Engineers.
The deleted scene that this conversation comes from is reported to be 10 minutes long. It's quite possible that a translation will be included somewhere on the DVD/Blu-Ray.
According to the consultant
In the scene David is learning the building blocks of language and we see him taking a lesson in Proto-Indo-European (PIE) Linguistics, where a holographic professor, takes him through the ABC’s and recites Schleicher’s Fable. An artificial text composed in the reconstructed PIE, in 1868, to demonstrate the language’s use.
hjew?s jasm? hwæln? nah?st ak?uns?z dad?kta (Translated as: a sheep that had no wool saw horses) – Excerpt from Schleicher’s Fable – The Sheep and the Horses"
The ‘Professor’ in the clip is in fact the real-life linguistics consultant used for the film and taught Michael Fassbender (David) the dialogue. I managed to track down the consultant, a Dr. Anil Biltoo of the SOAS Language Centre in London, to see if he could shed some light on the mysterious final scene. He was most helpful and provided the following:
The line that David speaks to the Engineer (which is from a longer sequence that didn’t make the final edit) is as follows:
/ida hman?m a? kja nam?tuh zd?:taha/…/gh??vah-pjorn-?ttham sas da:t? kredah/
A serviceable translation into English is:
‘This man is here because he does not want to die. He believes you can give him more life’.
(quote taken from this forum, which copy/pasted from the article)
I think David informed the Engineer that "this man" (Weyland) created him in order to get immortality. The Engineer found this idea crazy. Possibly, they created humans exactly for the same reason (as an experiment). So, the Engineer said to Weyland:
"Indeed we wouldn't create you if we were immortal!"
The whole idea is that artificial life and other smart tools ("life pods") can't help humanity in its struggle with death. More likely there is deadlock this way. It can make even worse. A tool can be used against its creator, like Weyland was killed by David's head.
The source? Probably, the oldest tale on the Earth. Take a look on a fragment from Epic of Gilgamesh (1700 BC):
In order to learn the secret of eternal life, Gilgamesh undertakes a long and perilous journey. He learns that "The life that you are seeking you will never find. When the gods created man they allotted to him death, but life they retained in their own keeping."