Answer from Quora:
I recently talked to a man who is learning Min Nan, which is a Sinitic language often called a dialect of Chinese. He told me that Min Nan speakers say that the tones are so hard that no one who doesn’t grow up speaking Min Nan ever seems to get it very well.
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 nine tones.
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 hard. Even Navajo children struggle quite a bit learning Navajo and don’t seem to get it well until maybe age 12. When Navajo children arrive at school, they often do not speak Navajo well yet.
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% accuracy, and everyone makes errors.
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!
Piraha, spoken in the Brazilian Amazon, is also very hard. Over the course of a few centuries, several Portuguese speaking priests had tried to learn Piraha, but they had all given up because it was too hard. And these same priests had been able to master a number of other Indian languages, but Piraha was just too much. Daniel Everett learned the language and wrote important papers on it. He is only of the only non-native speakers who was able to learn the language.
Tsez, spoken in the Caucasus, is also murderously hard. Every verb can have over 100,000’s of possible forms. I understand that even native speakers make regular errors when speaking Tsez.