My understanding of the Muslim belief about the Gospel of Jesus is that it has been largely lost, although pieces of it remain in the versions that modern Christians use. In what language was the Injil originally composed? Was it Hebrew? Aramaic? Perhaps Arabic?
The Injil could have been written in Aramaic or Biblical hebrew. but when it comes to current gospel it is a set of 4 injils some written in Greek others in Aramaic and epistles. There were many Injils circulating in the first centuries of christianity the current version has been canonised almost 4 centuries later.
It is a subject of a huge debate as lutheran bible does not accept certain passages.
http://www.catholiceducation.org/articles/religion/re0134.html (from a catholic perspective)
Some of those who wrote the gospels were not even contemporaries of Jesus.
The very word "Injeel" is derived from the Greek word "evangelion" and hence it is extremely unlikely that Jesus would have used this term to describe his own book. However, if one insists that Jesus used this term, then we would have to say that Jesus probably wrote either in Greek or in a Semitic language that was strongly influenced by Greek, as is shown by the very title of the book.