It's hard to find estimates of the difficulty of learning different (natural) languages, especially if one's first language is not English.
But the Foreign Service Institute of the US State Department once composed a list of languages with difficulty levels for the proficiency that they aim at, and I will summarize that list (source: Wikibooks:Language Learning Difficulty for English Speakers - Wikibooks, open books for an open world)
Category I: 23-24 weeks (575-600 class hours)
All Romance languages, all Germanic languages except English (0), German (I+), and Icelandic (II)
Category I+: 30 weeks (750 class hours)
German
Category I++: 36 weeks (900 class hours)
Indonesian, Javanese, Jumieka, Malay, Swahili
Category II: 44 weeks (1100 class hours)
Every language not in the other categories. Some of those listed require more time than this, and they may deserve a category of their own, Category II+.
Category III: 88 weeks (2200 class hours)
Arabic, Chinese, Japanese, Korean
Japanese is more difficult than the others.
I'll now consider what might contribute to these difficulties. Indonesian and Swahili one might expect to be type II languages, but they are somewhat easier. Very regular grammar? Chinese, Japanese, and Korean have plenty of Chinese characters, Korean also has an alphabet, and Japanese also two syllabaries. I can't see why Arabic isn't II or II+.
As to classical languages, I'd expect Latin, Classical Greek, and maybe also Sanskrit to be II or II+ (what Finnish and Hungarian are).
I've had zero success in finding similar lists for native speakers of languages other than English, though I'm sure that some people may have made some such lists. Like the Soviet KGB and its Russian successors (domestic: FSB, foreign: SVR).
But the Foreign Service Institute of the US State Department once composed a list of languages with difficulty levels for the proficiency that they aim at, and I will summarize that list (source: Wikibooks:Language Learning Difficulty for English Speakers - Wikibooks, open books for an open world)
Category I: 23-24 weeks (575-600 class hours)
All Romance languages, all Germanic languages except English (0), German (I+), and Icelandic (II)
Category I+: 30 weeks (750 class hours)
German
Category I++: 36 weeks (900 class hours)
Indonesian, Javanese, Jumieka, Malay, Swahili
Category II: 44 weeks (1100 class hours)
Every language not in the other categories. Some of those listed require more time than this, and they may deserve a category of their own, Category II+.
Category III: 88 weeks (2200 class hours)
Arabic, Chinese, Japanese, Korean
Japanese is more difficult than the others.
I'll now consider what might contribute to these difficulties. Indonesian and Swahili one might expect to be type II languages, but they are somewhat easier. Very regular grammar? Chinese, Japanese, and Korean have plenty of Chinese characters, Korean also has an alphabet, and Japanese also two syllabaries. I can't see why Arabic isn't II or II+.
As to classical languages, I'd expect Latin, Classical Greek, and maybe also Sanskrit to be II or II+ (what Finnish and Hungarian are).
I've had zero success in finding similar lists for native speakers of languages other than English, though I'm sure that some people may have made some such lists. Like the Soviet KGB and its Russian successors (domestic: FSB, foreign: SVR).