Yes, if you lose you will have to pay to repair his car.

Maximum time could be very long - depending upon when court is in session, if any medical certificates are needed, police reports, etc. [I had a court case (car accident, with personal injury) that took nearly 2 years to complete -- because the other guy kept NOT showing up to court, so they'd postpone things for a month, then he wouldn't show again...]

Don't get a lawyer. You don't need one. If you have a friend who speaks Arabic to help you, that will help things go more smoothly.

Yes, you can leave the country while the case is still in the system/undecided. Sounds like a relatively low-key car accident case, nothing that would cause you to be forced to stay in country.

Nationality shouldn't matter...but as I said, make sure you or a friend is there to speak/explain things in Arabic. The court-appointed interpreter often leaves a great deal to be desired...so bring your own to ensure that YOUR side of the story is adequately explained.