What is the most difficult language to learn?

How difficult a language is to learn is largely dependent on what language(s) you currently speak, since languages with more similarities will be easier for you to learn. The US State Department has curated a list of languages based on how difficult they are to learn for English speakers. The most difficult languages in the list are classed as "Category III", which include Arabic, Cantonese, Mandarin Chinese, Japanese and Korean.