Indigenous languages are the most difficult to learn.
Broadly speaking, languages with the fewest number of speakers are the most complex and most difficult to learn. Not only are these languages difficult to access and may not have a writing system, but these languages also become the most complex when they are isolated and spoken by fewer people.
Although their native speakers only make up around 6% of the world’s population, there are over 4,000 indigenous languages currently spoken today. However, even the most widely spoken indigenous language - Southern Quechua - is only known to around 6-7 million native speakers. The first major obstacle you would have to overcome to learn an indigenous language is finding a way to learn it; while there are plenty of programs and tutors available for more widespread languages such as English, Spanish, and French, most indigenous languages are concentrated geographically in smaller communities. Duolingo is currently the only language-learning software that offers lessons in two indigenous languages - Navajo and Hawaiian. Indigenous languages are highly unique and can be extremely complex in a number of ways, such as pronunciation, grammar, sentence structure, irregularities, etc. For example, the eastern Amazonian language Tuyuca uses evidentiality, meaning that it “requires verb-endings on statements to show how the speaker knows something”. Silbo Gomero, spoken in the Canary Islands, is a language composed entirely of whistles. Some of these languages are isolate, meaning that they have no known relationship to any other language; so, no matter what a person's first language is, they would have a lot of difficulty learning a language isolate. Lastly, many indigenous languages do not have a writing system; while indigenous peoples have worked with linguists in the past to develop a written form of their language, the form may not even be agreed upon by members of the community. For example, the Shoshone language has developed multiple separate writing systems mainly for this reason. It is much more difficult to learn a language orally when not completely immersed in that native environment - especially without the guidance provided by a concrete writing system.
Rejecting the premises