What’s the most difficult language in the world?

Learning a language is difficult. Everybody who studied a language in the past knows that. You might not only come across letters or words, which are virtually impossible to pronounce for you, sentence structure and grammar might also differ drastically from your native language. Therefore it also depends on what your native language is, how difficult it is for you to learn another language. For example, a Spanish person will obviously have a much easier time learning Italian, compared to a Chinese person for example. Francis Xavier, an important Jesuit missionary to Japan in the 16th century once stated, that Japanese is the language of the devil. A sentiment that reflected his difficulties in learning the language and that could easily be understood from his perspective, since Japanese writing or spoken language had absolutely nothing in common with any European language of the time. Today Japanese has at least adopted many loanwords from European languages (such as "aisu kurimu" for ice cream, "baiku" for motorcycle, "resutoran" for restaurant and so on). On the other hand, a linguist once said, that if Martians landed on planet earth and he had to advise them which language would be easiest to learn within a short period of time, he would suggest Turkish to them, since it has a very well structured regular grammar, that is easy to memorize compared to other languages. And if you already speak Arabic or French, you have a head start. About 4000 Turkish words are of French origin and between 40-50% are of Arabic origin.
But there are also some distinct criteria, which make some languages stand out. According to the Population Reference Bureau, which took a “guesstimate” in 2002, about 100,000 languages have existed throughout the history of mankind, of which 6000 are still spoken today.
Some only by a handful of people, some by hundreds of millions of people and of course not all of those can be included in this list, but here are some that come to my mind (in no specific order).

Xhosa
“It’s not a noise, it’s a language”, as some people would say. Xhosa is a language spoken by about 19 Million people in South Africa, Lesotho and Zimbabwe. It is defined by click sounds, which are difficult to impossible to learn if you didn’t learn the language from an early stage on. Xhosa has 18 different click sounds. There are two other languages with more click sounds, one of which would be Jul’hoan, spoken by about 10,000 people in Botswana and Namibia, with about 48 click sounds. The other one would be Taa, spoken by about 4,000 people in Botswana with about 83 click sounds (the largest inventory of consonants of all languages in the world).

Check out a tongue twister in Xhosa:

So next time you complain about difficult French pronunciation or harsh German consonants, remember, it could be way way worse.

Arabic
Since I spent many years studying Arabic myself, this is a subject I can speak on out of experience. Studying Arabic is hell. For the average European, Arabic is a mess. Not only that they write in the “wrong” direction (from right to left), their grammar and pronunciation is quite a challenge too. Arabic has only three vowels, a, i and u and many times they are not written down, so you have to know the words, in order to know how to pronounce them.
Using a dictionary is not easy too, because of the way the Arabic language is set up. Arabic language, just like all Semitic languages, uses root consonants, as a base for all their words.
For example, if you want to find the word "maktab" (desk), you have to know, that the consonantal root for the word is "k-t-b/kataba" (to write). So instead of looking up words that start with "m", you have to look for words that start with "k". So "maktab" is something that is used to write upon. It can also mean school (i.e. a place where people write). A "katib" is a writer…And so on. This goes for pretty much all of the words.
Confusing? You got that right!
Another difficult thing about Arabic is its harsh pronunciation. You have several versions of the letters "k", "s" or "t", which take some practice to pronounce. For example” ﻙ “ is pronounced like a regular k. But “ﻕ“ is pronounced like a "k" when you cough up candy, that got stuck in your throat.
I have met people who couldn’t do it all and quit their studies.
But Arabic has had its fair share of influence on other languages too. Some Arabic words in English are: safari (literally means journey), algebra, alcohol, admiral (derived from the word Amir/Emir i.e. meaning commander), coffee, mattress (derived from the root "tarah", which means to throw something, therefore a mattress is something that is thrown/put on the ground), traffic (from "tafriq", meaning distribution), tariff, tobacco, zero etc. The list is long.
So before you attempt to take on this beast of a language, make sure you have a lot of time on your hands and travel there for practice.

Chinese
You probably might as well put any other character-based East Asian language here, but since Chinese is the most commonly spoken language in the world, I’ll go with that. Chinese doesn’t use letters but characters. There are up to 100,000 characters (there is probably not a single living person that knows all of these), even though you “only” need to know 900 characters to be able to read about 90% of a newspaper and have everyday conversations.
Chinese is a tonal language. This means the way you say a word can entirely change its meaning. Depending on how you pronounce a word, it can turn the sentence you want to say into confused sounding gibberish. Words which sound the same, but have a different meaning are called homophones, so these are near homophones. For example, "xióngmāo" means Panda. But "xiōngmáo" (note the different diacritic sign above the letter a, which expresses a different tonality) means chest hair. If you go to the zoo in China, you better not ask the clerk where the chest hair is. ;)
Mastering the four tonalities of Mandarin Chinese has been called one of the supreme challenges for non-Chinese people trying to learn the language. But you could put your foot in it even worse:
"xiǎo jie, shuǐ jiǎo yī wǎn duō shao qián" means: "Miss - how much is a bowl of dumplings?"
While "xiǎo jie, shuì jiào yī wǎn duō shao qián?" means: "Excuse me miss, how much does it cost to sleep [with you] for a night?"
So you better be careful with that tonality! ;)

Of course, then there is the problem of cultural differences on top, which makes it impossible to translate certain grammatical features or words. Arabic has two versions of the plural form. One for everything that comes in two units (it is called Dual) and one for everything that comes in more than two units (E.g. sanatun - one year, sanatāni – two years, sanawātin – 3+ years). And Turkish has the awesome word of "yakamoz", which roughly translates to: "the moonlight which is reflected on the surface of the water". There is also a pretty legendary word in the Finnish language, "Kalsarikännit". It roughly translates to: "Getting drunk at home alone, only in your underwear, with no intentions of going out". Yes, this really exists.
Now try to put that into one English word….;)

Here's an Italian having a hard time with English...;)

What difficulties have you had while learning languages? Which language is your favourite pain in the neck? What language would you like to learn?