I was about to write my next post on prosody when I came across your article, what a great read! I have been learning about 10 languages so far but have never tried Arabic, definitely would like to do it in the near future! I have had some experience learning Chinese and Thai, though, and I must say that learning tones in any of these languages is not as difficult as it seems. It definitely won't happen overnight but from my observation most learners don't do it the right way, which is practising the tones every day, for at least a few weeks. All the mystery and magic surrounding tones fades away once proper work is done!

I would also add Polish to the list of the most difficult languages. I have read numerous articles by other linguists and, to cut the long story short, it seems that after a few months of learning Mandarin you will be able to communicate more freely than after a few months of learning Polish. It's connected to its grammatical complexity, lots of exceptions and pronunciation of Polish language. The idea doesn't take writing into account, of course. Looking forward to your next post!