The Foreign Service Institute (FSI) is the primary training institute for employees of the “US foreign affairs community” (diplomats, military personnel, etc.). The FSI is probably most well known for its foreign language courses, which, while sometimes a bit outdated, is still great quality and in the public domain.
The FSI has has compiled “approximate learning expectations for a number of languages based on the length of time it takes to achieve [professional working proficiency]”.
Reddit’s /u/Fummy took that data and created the maps below, showing the learning difficulty of the major European languages and the major Eurasian and North African languages:
The underlying data comes from Wikibooks, where the following disclaimer is important:
It must also be kept in mind that students at FSI are almost 40 years old, are native speakers of English and have a good aptitude for formal language study, plus knowledge of several other foreign languages. They study in small classes of no more than six. Their schedule calls for 25 hours of class per week with three or four hours per day of directed self-study.