What is the most difficult language in the world?

The difficulty of a language depends on many things. Native speakers of English can learn Spanish with less difficulty than Native Korean speakers, since English is closer to Spanish than Korean is.

On top of that some languages are considered easy to learn to speak, but difficult to learn to write and vice versa. An example would be Mandarin Chinese, from the perspective of Native English speakers it is considered easy to learn to speak, but difficult to read and write.