The typical crazy view in my field, Linguistics, is that no language is harder to learn than any other language. It is true that it depends on where you are coming from. Japanese and Korean speakers find it easy to learn each other’s languages. A Russian can learn Polish fairly easily, and a Faroese can learn Icelandic without major problems. Indians in the Vaupes region of Colombia regularly learn 4-5 of the hardest languages on Earth, often in adulthood. If you come from a language that has familiar features with the language you are trying to learn, the L2 is going to be a lot easier for you.
Nevertheless, there do seem to be some languages that are just hard to learn.
A language with a greater amount of irregularity and a more complex system would seem to be much harder to learn.
Things making a language hard to learn include: multiple inflection classes, noun classifiers (the worst ones have dozens of classifiers), a gender system, especially an unpredictable one, a large phonemic inventory, unusual or many unusual sounds, distinctions between aspirated, non-aspirated and glottalized consonants, voiceless nasals and glides, tones, tone terracing, vowel distinctions in length with up to three grades, breathy-voiced, creaky-voiced, murmured, nasal and semi-voiced vowels, vowel and nasal harmony, unpredictable pitch, accent and stress systems, complex morphophonology, many different different forms, irregular verbs and nouns, many tenses, voices and aspects, grammatical honorific systems, articles… a complicated deictic system, many cases, split ergativity, singular, dual, trial, plural and the like, a difficult orthography, especially one with a poor sound-symbol relationship, logographics, thousands of characters, multiple letters representing the same system, or orthographies were more than 2-3 systems are combined into one.
On the other hand, creoles and languages like Indonesian and Malay are said to be very hard to learn as they are greatly simplified. For instance, Indonesian lacks most of the above.
Doesn’t it seem obvious that Indonesian is easier to learn than ǃXóõ?
