What is the most difficult language do you think on Duo ??
Even though I haven't studied it at all, I'd say Japanese, since it's the only Asian language on Duolingo whose alphabet uses a complete different system made up of symbols! There's also Hebrew, Greek, and Russian, which all use different alphabets, Hebrew's alphabet being the most different from the Latin alphabet of the three, but Japanese is a whole different, much harder story. :)
But then again, each person is different. :D
Hungarian, Hebrew and Vietnamese. Hungarian is probably the hardest.
I've only really studied a few languages on Duolingo past the basic lessons and those are: Portuguese, English, Spanish, French, Italian and Swedish. I've tried doing a few lessons for almost any other tree except for Swahili and Welsh or something like that and I will base my opinion on my initial perception of all the languages.
The hardest languages on Duolingo for me are: Hungarian, Hebrew and Vietnamese. These trees must be extremely difficult to complete. Only doing the basic lessons seemed like a nightmare and something I would never be able to do. There are a few factors fo this and I think the main one is a lack of motivation. Neither of these is an Indo-European language, Hungarian has an extensive case system and "weird" word order, Vietnamese has a lot of tones and I couldn't get a grasp of Hebrew whatsoever. Hebrew seems to be the easier to learn because it was designed to be easy to learn, I just lack motivation for now.
Yes, I too lack motivation for Hebrew now. That's why I've deleted it. And a few other languages, but it's fine because sometimes simplifying now is way better so that way you can leave room for more things, or languages, later on in life, when and if you're truly ready for more! :)
The Hebrew course, between the script and a high level of unfamiliarity (relatively little borrowing or vocabulary sharing with European languages), plus the lack of TTS so you don't get words in isolation, makes the initial learning curve for Hebrew really, really tough. It's easy to get discouraged in those early stages. Once you're over that hump, it gets much easier. A lot of the difficulty of Hebrew is that it's different, not that it's inherently complex.