Which languages are the most difficult to learn?

The Defense Language Institute, one of the USA's top language schools, categorizes languages into four difficulty levels based on the amount of class time they've found it takes native English-speaking students to reach a particular level of fluency.

• Category I languages, 26-week courses, include Spanish, French, Italian and Portuguese.
• Category II, 35 weeks, includes German and Indonesian
• Category III, 48 weeks, includes Dari, Persian Farsi, Russian, Uzbek, Hindi, Urdu, Hebrew, Thai, Serbian Croatian, Tagalog, Turkish, Sorani and Kurmanji
• Category IV, 64 weeks, includes Arabic, Chinese Mandarin, Korean, Japanese and Pashto

(5) What is the easiest spoken language to learn? - Quora