There is a fairly standard ranking of the difficulty of learning various languages for English speakers, based primarily on its similarity to English. (Note: I've seen some contradictory information about the list). Some of them are fairly clear (e.g. Spanish, French, and Italian are Category 1 - i.e. "Languages closely related to English," considered relatively easy for English speakers to learn). Category 1 languages are estimated to take 23–24 weeks (575–600 class hours) of study to learn.
A Category III Language is defined a language which is "quite difficult for native English speakers" with an estimated learning time of "88 weeks (2200 class hours) (about half that time preferably spent studying in-country)".
There are several other categories (Category II and Category IV) as well depending on the estimated time and difficulty of learning them. Most of these are what you'd expect, too - for example, Mandarin Chinese is regarded as a Category IV or Category V language (depending on which source you read) - i.e. extremely difficult for an English speaker to learn.
Some categorizations are very odd, though. For example, Dutch and Afrikaans are Category 1 languages (relatively easy for English speakers), but German is a Category 2 language. This seems a little arbitrary to me. Based on the time estimates for learning the languages, this means that the list compilers expect that it will take at least 25% longer to learn German than it take to learn either Afrikaans or Dutch (750 hours of classroom instruction vs. 575 - 600).
How did they determine this? For example, why do they think that German is 25% harder than Dutch? Was this list based on actual statistical studies? If so, how were the studies conducted?
Also, do they rank rare languages (e.g. the Navajo language)? How did they decide which languages to rank for the list and which languages to leave out? Has anyone ever tried to apply the same methods to any of the languages that the FSI didn't study?