A question was recently asked on Quora. Here is my answer.
Hello, I recently talked to a Westerner who is learning Min Nan, which is a Sinitic language often called a dialect of Chinese. He already speaks Mandarin, but he told me Min Nan if vastly harder than Mandarin. At age 35, he was studying it 2 hours a day, and at some point, he hit a wall, and he didn’t seem to be making any progress. He kept adding more study hours to the day – four hours, six hours – with little effect. Finally when he was studying it for eight hours a day, he started making some good progress. I believe he said contour tones and tone sandhi were the major roadblocks.
Min Nan speakers say that even Cantonese is easier than Min Nan, and Cantonese is deadly hard. They also say that Min Nan tones are so hard that no one who did not learn Min Nan growing up gets anywhere near native fluency.
Cantonese is a similar language that is very difficult. It is much harder than Mandarin, and many native Mandarin speakers say they tried to learn Cantonese and gave up on it because it was too hard. Cantonese has 9 tones. The general consensus among Chinese is that Cantonese is much harder to learn than Mandarin.
Basque is said to be very hard to learn unless you grow up with it. There is a joke that the Devil spent seven years trying to learn Basque, and he only learned how to say Hello and Goodbye.
Navajo would also be murderously hard. Even Navajo children struggle quite a bit learning Navajo. When they show up at school at age 5-6, they are still struggling with Navajo. There are reports that Navajo children don’t seem to get Navajo well until maybe age 12.
Korean is a surprise, but apparently it is very hard to learn well. A native Korean speaker told me that Korean is so hard that no Korean speaker ever speaks it with 100
As another respondent pointed out, Japanese is also quite notorious, and most Westerners get nowhere near native fluency.
Czech is also hard. Even most Czech speakers never get Czech all the way. They have TV contests in Czechoslovakia where they try to stump native speakers with hard forms in the language. If you can last 30 minutes without making even one error, you win. I think only two men have been able to do it, but one was a non-native speaker! Czech also has a strange r sound found only in one other language on Earth. It is said that no native speaker ever gets this phoneme quite right.
Piraja is also very hard as another respondent pointed out. Only two non-natives have ever been able to speak Piraha with any fluency. When Daniel Everett went to study the language, he found a number of reports from priests who had tried to learn Piraha since the early 1800’s, and only one had succeeded. The others tried to learn but gave up because they said it was too hard.
Tsez, spoken in the Caucasus, is also murderously hard. Every verb can have tens of thousands of possible forms. Reports say that even native speakers make regular errors when speaking Tsez.