Would love to use duolingo, but they have no interest in supporting iranian languages. I’ve never really seen a clear answer for why. They have about 200 million native speakers worldwide. Persian, Pashto, Kurdish are the largest.
But it's harder because the writing system is tricky. They teach arabic and probably that was already pretty difficult to implement. Persian will also need customizations similar to arabic.
(For the record, I’m not saying the above languages shouldn’t be included, rather that the argument doesn’t hold up)