In conclusion, I reckon that to say any language is the hardest to learn in the world is just silly.
Not really silly if you know about the psychology of language skills. To start with, any language is easy for a native speaker. Then there are 2nd, 3rd and so on languages. Awareness and skills of languages other than your native language are a huge step in making subsequent languages easier to learn since, to simplify things, your mind has been trained, achieving a degree of mental flexibility, and the flexibility of the organs of speech. You have that experience.
The issue then is not just how hard a language is in isolation, but how hard it would be in comparison to other languages which are not one native ones. The difficulty in "unlearning" the old, rather than learning the new, if you will. Much like driving a car. My first car was a standard. I had quite a ride trying to unlearn that and drive an easier to handle automatic instead.
Still, there are languages with certain complexities not present in one's native language. Often those complexities are not subject to simple translations but are the result of hundreds, or thousands of years of social development, intricacies etc. On the basic level it's not hard to communicate in many languages after just 1 month of intense study. I tried with English. All it really takes is less than 2000 words, some basic grammatical structures and you can get by, defining word you don't know using the words you do know. Try to communicate in Polish using 2000 words and "basic" grammatical structures. No way Jose. I experienced that too. The Polish texts were incomprehensible in writing.
I also heard Polish spoken by "quick learners" (Radio Tirana). It took me about 10 minutes of intense listening to even realize I was listening to a broadcast in, what turned out to be Polish. That wasn't poor Polish, or incorrect Polish. That was simply gibberish.