They don't have an Arabic course yet but courses for Arabic speakers. I just tried out German for Arabic speakers. It was funny to do it in reverse.
Great, that you're happy with Duolingo! :)
I haven't made any progress in Russian, so that's why I'm resorting to Duolingo now. I'm thinking now about my strategy because completing the Greek tree brought me nowhere. I just don't know how to approach Russian. Writing texts also didn't bring me anywhere. Talking was disastrous. I couldn't make myself continuing with my textbook nor listen to a podcast course. I'm just trying to do whatever it takes to make Russian stick. I'm using Duolingo and LingQ now and websites with graded stories in Russian. There must be a way for me to learn this language and I'm going to find it.