Apparently, this is a really popular subject online recently. I have no idea why. It’s mostly a subjective question. But I did some quick googling for actual ‘facts’. The best information I could find was:
‘The Foreign Service Institute (FSI) of the US Department of State has compiled approximate learning expectations for a number of languages. Of the 63 languages analyzed, the five most difficult languages to reach proficiency in speaking and proficiency in reading (for native English speakers who already know other languages), requiring 88 weeks, are: “Arabic, Cantonese, Mandarin, Japanese, Korean“, with Japanese being the most difficult.’
Nonsense. I bet they never even tested Hungarian. Hungarian is way super harder and a lot less useful (bocsi 
)
Then I’ll throw in a little more subjective drivel I also found on some random blog (be careful, they may try to sell you stuff):
‘Extremely Hard: The hardest language to learn is: Polish-Seven Cases, Seven Genders and very difficult pronunciation. Very Hard: Finnish, Hungarian, and Estonian-These languages are hard because of the countless noun cases Pretty Hard: Ukrainian and Russian complex grammar and different alphabet but easier pronunciation. Fairly Hard: Chinese and Japanese-No cases, no genders, no tenses, no verb changes, short words, very easy grammar, however, writing is hard. But to speak it is very easy.’
Also nonsense. I learned Hungarian pretty well, and I have no idea what noun case is, nor do I care. They failed to mention that Polish is still relatively Slavic and follows slavic grammatical rules and is easy to pick up if you know any of the 20 Slavic languages. Or how Finnish-Estonian-Hungarian follow grammar rules similar to nothing.
And I picked up writing and reading in Japanese before I started speaking it.
The real reason for any language to be hard to learn is if there is enough interesting material to drive you to learn it in the target language. Hungarian has… 1 or 2 really funny native movies (Uveg Tigris. Valami Amerikai) which you can understand at an intermediate level. There are also a few hundred excellent books, but they require a fluent-native level to enjoy. There is nothing else. Children’s books are either boring or more difficult to read than adults books (I’m serious, and I know the reason). Everything interesting at different levels has either been translated from English media (which, you know already) or some other language (which, well, go learn that language then). So learning Hungarian is an excerise in frustration.
Japanese on the other hand is ripe with tons of interesting native media to enjoy at all levels. Manga, Anime, Video Games, Movies, you name it, they’ve got it.
So basically, hardest language to learn is the most boring language. (I’m voting for Estonian 
)
Honestly, it’s a stupid question in the first place. If you’re asking this question, you’re not looking to learn, you’re looking for some sort of language appreciation… or an excuse not to learn.
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{ 8 comments… read them below or add one }
Finnish is not hard.. i dont get it why alot of sites say its hard..
Perhaps because the grammar is Finno-Ugric which is very difficult to compare or learn for most western languages. Similar to Hungarian.
Great post. Chinese is not hard as well. I have a free Chinese learning website for people who are keen to learn Chinese.
Nice site, reminds me a bit of all japanese all the time.com, but of course with Chinese instead of Japanese
Hi I would say language learning is infinatley easier when you are activley interested in learing the language.
Here is my Mandarin Learning blog.
Hi Charlie, thanks for stopping by
I completely agree with you that a language is a truckload easier if you are actively interested in it. I think that finding exciting target language materials (comics, video games, books, movies, etc) is key to staying interested. Slick mandarin site btw! 
Thanks, there is plenty I am still not happy with the site, but always the way when you set one up I guess.
I think that having the blog keeps me motivated to keep learning, because learning on my own is tough not having anyone to bounce it all off. Also makes me feel guilty when I have nothing to talk about on the blog if I have been slacking.
THe topics I cover help me learn other aspects too. I am finding it a very interesting journey.
I bet
Nothing motivates/scares me more than doing SRS reps every day, because if I don’t do them one day, there will be twice as much work to do the next!