Which language is the most difficult to learn in your opinion (and why)?

I have heard that the most difficult language to learn is English. However my native language is English so it is hard for me to say that is the most difficult language to learn since I already know it. I have heard that English has a lot of slang words and phrases in it and it also has figurative language that is difficult to understand.

The English language also has many rules to learn and they have many exceptions to the rules and those have to be learned as well. A language that I find hard to learn that is not English might be Swahili or another language that is not commonly taught. It would be hard are to learn this type a language because there are not a lot of resources out there to learn from.