Arabic is in the works.
All the courses you can learn are here: https://www.duolingo.com/courses
To also see those being created look here: https://incubator.duolingo.com
Hebrew is related to Arabic and is available on Duolingo.
Arabic is under construction right now.
It will be a long time before the Arabic course is finished, but I'm so glad at least now we know that eventually there will be one.