What is the hardest language to learn?

After 70 years of experiencing teaching languages to American diplomats, the U.S. Foreign Service has grouped foreign languages into four categories of difficulty. The easiest require 575-600 hours of study (23-24 weeks of classroom study) for students to achieve sufficient competence to be posted overseas, whereas the hardest group requires at least 2,200 hours of study (88 weeks of full-time classroom study) to achieve the same level of proficiency. In other words, some languages can be 3-4 times harder to master than others.