Chinese has been widely considered to be one of the most difficult languages in the world. What constitutes the difficulty of a language? Can it be measured and how? Whenever someone posts a message about language difficulty on a forum, it almost always generates a heated discussion. Comments range from "English is the easiest because the verbs have minimum conjugations and nouns have no gender", "Chinese and Japanese are hard because there're too many characters or kanji's", to "No language is inherently more difficult than any other because native speakers grow up speaking it with about the same effort", and "Language difficulty is subjective perception", to name a few.
Most language enthusiasts on various forums are not scholars. The diversity of those opinions is a result of no good definition of language difficulty. But we can tell that most people are referring to the difficulty experienced by an adult (not a young child) in learning a foreign language (not mother tongue), and in many cases the adult's native language is English. If we qualify the discussion with these requirements, i.e.
- the learner is an adult;
- the language whose difficulty is evaluated is learned by the adult as a foreign language;
- the difficulty is evaluated when the adult's native language is specified
I believe that in many social sciences, there are two general methods to measure a quantity, internal and external. For example, in linguistics, a researcher can define a set of factors pertinent to the correlation between orthography (spelling) and pronunciation in order to calculate the orthographic depth of a language, i.e. "the degree to which a written language deviates from simple one-to-one letter-phoneme correspondence". Alternatively, one can simply conduct a controlled study among a group of people (cohort) and see which language causes how many spelling errors in dictation or in a similar experiment.
When it comes to rating language difficulty, we can devise a set of rules and individually assess each language against these rules and then sum the rule ratings (with weights); e.g., percentage of words that have cognate or loan relationship with the words in the learner's native language, whether the nouns have genders and cases, how many variations in verb conjugation, whether the dominate word order differs from that of his native language, etc. But so far I'm not aware of such internal research, even though it's doable.
The external evaluation, on the other hand, has been done and is widely quoted. The most well-known data for English native speakers are from Defense Language Institute of the US, where they statistically measure the time for the learners to take in achieving a certain language proficiency level. The official Web page for this study is https://www.ausa.org/articles/dlis-language-guidelines, duplicated below for your convenience.
- Category I languages, 26-week courses, include Spanish, French, Italian and Portuguese.
- Category II, 35 weeks, includes German and Indonesian
- Category III, 48 weeks, includes Dari, Persian Farsi, Russian, Uzbek, Hindi, Urdu, Hebrew, Thai, Serbian Croatian, Tagalog, Turkish, Sorani and Kurmanji
- Category IV, 64 weeks, includes Arabic, Chinese Mandarin, Korean, Japanese and Pashto
Hours of instruction required for a student with average language aptitude to reach level-2 speaking proficiency
|Speaking proficiency level expected of a student with superior language aptitude, after 720 hours of instruction|
|GROUP I||Afrikaans, Danish, DUTCH, FRENCH, Haitian Creole, ITALIAN, Norwegian, PORTUGUESE, Romanian, SPANISH, Swahili, SWEDISH||480||3|
|GROUP II||Bulgarian, Dari, FARSI (PERSIAN), GERMAN, (Modern) Greek, HINDI-URDU, INDONESIAN, Malay||720||2+ / 3|
|GROUP III||Amharic, Bengali, Burmese, CZECH, Finnish, (MODERN) HEBREW, Hungarian, Khmer (Cambodian), Lao, Nepali, PILIPINO (TAGALOG), POLISH, RUSSIAN, SERBO-CROATIAN, Sinhala, THAI, TAMIL, TURKISH, VIETNAMESE||720||2 / 2+|
|GROUP IV||ARABIC, CHINESE, JAPANESE, KOREAN||1320||1+|
I've found one possible source about the difficulty in learning a few languages as a foreign language when the adults' native language is NOT English, a book by Robert Marzari, Leichtes Englisch, schwieriges Französisch, kompliziertes Russisch. The book content is unavailable on either Amazon or Google Books; hence my "possible source". Without reading the book, I interpret the book title as English < French < Russian in order of difficulty (which may be intuitive to most polyglots). And it's not clear whether that order is for all Europeans in general (i.e. average) or for a specific native-language group.
Unfortunately, I'm not aware of any other research on this topic. But as you can already see, an otherwise hot topic can be made quite cool by the above analysis, cool as opposed to hot or debatable, and cool in the sense of being interesting.