This question was posed in one the emails I received today. I managed to find some information about the relative difficulty of learning particular languages for English speakers, but not for speakers of other languages.
The difficulty of learning a particular language depends on which language(s) you already know. Each language presents you with a different set of challenges, including differences in pronunciation, vocabulary, grammar, spelling and writing system. Generally the more differences there are, the harder a language is to learn, though there isn’t necessarily a simple correlation between interlingual distance and the difficulty learning.
Most people seem to think that Japanese and Chinese (any variety) are very difficult languages to learn. Having studied both I can confirm this. When learning these languages, the biggest challenge you face is reading and writing them.
Chinese grammar is straightforward; the pronunciation is not too difficult, though the tones take a lot of getting used to. It takes quite a long time to build up enough vocabulary to be to have more than a basic conversation, but the more words you learn, the easier it gets to learn new ones. Most of the vocabulary is constructed from native roots and there are very few foreign loanwords.
Japanese grammar is more complex than Chinese, though a less complex than most European languages, apart from the intricate politeness registers. Japanese pronunciation causes few difficulties, though the irregular intonation is quite a challenge. Japanese vocabulary is a mixture of native words and words borrowed from other languages, particularly Chinese and English. The English loanwords are all changed to fit Japanese phonology, and are often abbreviated and combined with native and/or words from Chinese. As a result, they are difficult to recognise as words that were originally English.
The Defense Language Institute in Monterey, California, classifies the languages they teach into four groups based on the number of hours of instruction (English-speaking) students need to attain a certain level of proficiency. In this scheme, the most difficult languages are Arabic, Chinese, Japanese and Korean. For more details, see:
http://www-personal.umich.edu/~wbaxter/howhard.html
