Method and Conclusion. See here.
Results. A ratings system was designed in terms of how difficult it would be for an English-language speaker to learn the language. In the case of English, English was judged according to how hard it would be for a non-English speaker to learn the language. Speaking, reading and writing were all considered.
Ratings: Languages are rated 1-6, easiest to hardest. 1 = easiest, 2 = moderately easy to average, 3 = average to moderately difficult, 4 = very difficult, 5 = extremely difficult, 6 = most difficult of all. Ratings are impressionistic.
Time needed. Time needed for an English language speaker to learn the language “reasonably well”: Level 1 languages = 3 months-1 year. Level 2 languages = 6 months-1 year. Level 3 languages = 1-2 years. Level 4 languages = 2 years. Level 5 languages = 3-4 years, but some may take longer. Level 6 languages = more than 4 years.
American Indian Languages
American Indian languages are notoriously difficult to learn, though few try to learn them in the US. In the rest of the continent, they are still learned by millions in many different nations. You almost need to learn these as a kid. It’s going to be quite hard for an adult to get full competence in them.
One problem with these languages is the multiplicity of verb forms. For instance, the standard paradigm for the overwhelming number of regular English verbs is a maximum of five forms:
Many Amerindian languages have over 1,000 forms of each verb in the language.
The Salishan languages are maddeningly difficult languages for English speakers to learn. Yet the Salishans always considered the neighboring language Kootenai to be too hard to learn.
Kootenai has a distinction between proximate/obviate along with direct/inverse alignment, probably from contact with Algonquian. However, the Kootenai direct/inverse system is less complex than Algonquian’s, as it is present only in the 3rd person. Kootenai also has a very strange feature in that they have particles that look like subject pronouns, but these go outside of the full noun phrase. This is a very rare feature in the world’s languages. Kootenai scored very high on a weirdest language survey.
Kootenai is an isolate spoken in Idaho by 100 people.
Kootenai is rated 6, hardest of all.