Extremely Hard: The hardest language to learn is: Polish – Seven cases, Seven genders and very difficult pronunciation. The average English speaker is fluent in their language at the age of 12, in contrast, the average Polish speaker is fluent in their language after age of 16.
Very Hard: Finnish, Hungarian, and Estonian – The Ugric languages are hard because of the countless noun cases. However, the cases are more like English prepositions added to the end of the root word. However, anyone arguing Asian languages like Korean trump Uralic languages in complexity, really needs to hit the books and do more research.
Simply Arduous:Ukrainian and Russian – Second language learners wrongly assume because these languages use a different script (Cyrillic) that it out ranks Polish. This is not objective, as an alphabet is only lets say 26 letters. It is really the pronunciation and how societies use the language that influences ranking. Ukrainian and Russian complex grammar and different alphabet, but easier pronunciation. (the Poles use a modified Latin alphabet which does not have a neat orthography fit to the sounds of their language). Slavic languages have sophisticated case and gender systems, also something that approximates a complex tense system with aspects of time-verb relationships.
Challenging contender jockey for position: Arabic - Three baby cases which are like a walk in the park compared to the above, but the unusual pronunciation and flow of the language makes study laborious and requires cognitive diligence if you want to speak it.
Fairly Hard: Chinese and Japanese - No cases, no genders, no tenses, no verb changes, short words, very easy grammar, however, writing is hard. But to speak it is very easy. Also intonations make it harder, but certainly not harder than Polish pronunciation. I know a Chinese language teacher in NYC that has even authored an the authoritative book on modern Mandarin says people pick up Chinese very easy. This same teacher, if multilingual yet could not learn Polish. I am learning some Chinese, it is not the hardest language maybe even one of the easiest language to learn. Despite prideful proclamations of armchair linguists, to verbalizes Asian languages in general are not top ranked by any measure. Try to learn some Chinese and Polish your self and you will see which is the hardest language.
Average: French - lots of tenses, but not used and moderate grammar. German-only four cases and like five exceptions, everything is logical, of course.
Easy: Spanish and Italian - People I know pick these up no problem, even accountants and technical people rather than humanistic language people.
Basic to hard: English, no cases or gender, you hear it everywhere, spelling can be hard and British tenses you can use the simple and continues tense instead of the perfect tenses and you will speak American English. English at the basic level is easy but to speak it like a native it’s hard because of the dynamic idiomatic nature.