Pǔtōnghuà (Chinese Mandarin)
If someone had tried to warn me how difficult this language is before, I would not have believed it. Chinese (by which is usually meant and understood Mandarin) is consistently ranked as the most difficult language to learn for English speaking natives, and the corollary is true for Chinese speaking natives. But I'm neither, and that gives me hope.
In short, it's a tonal language, and that makes speaking it very difficult for someone like me. It's also quite rich in homophones and full of idioms and aphorisms. Grammar, on the other hand, seems incredibly simple. The other major difficulty is its alphabet, mostly because it is very different (like all from that geographical area, some which are based on it) from what I grew up with.
Tones are the most difficult part of learning, at least so far. Here's from Wikipedia:
Standard Mandarin Chinese, the official language of China, has four lexically contrastive tones, and the numerals 1, 2, 3, and 4 are assigned to four tones. Syllables can sometimes be toneless and are described as having a neutral tone, typically indicated by omitting tone markings. Chinese varieties are traditionally described in terms of four tonal categories ping ('level'), shang ('rising'), qu('exiting'), ru ('entering'), based on the traditional analysis of Middle Chinese (see Four tones); note that these are not at all the same as the four tones of modern standard Mandarin Chinese. Depending on the dialect, each of these categories may then be divided into two tones, typically called yin and yang. Typically, syllables carrying the ru tones are closed by voiceless stops in Chinese varieties that have such coda(s) so in such dialects, ru is not a tonal category in the sense used by Western linguistics but rather a category of syllable structures.
To learn Chinese I first have to learn "Pinyin".
When I wrote that grammar is simple, I meant:
Word order is always Subject-Verb-Object
Verbs have only one dictionary form and no other conjugated forms.
Nouns do not require any articles.
"He" and "she" are pronounced the same (ta [level]) but plural is marked with men [rising].
In terms of culture, my impression is that, like its neighbours, Chinese society is highly deferential and respectful of authority, although I would expect communism to have reduced some of that. Partly because of it, networking and personal connections are more important than in Western countries; there's even a word for it, guānxi (关系).
Learning Chinese taught me to appreciate the effort Chinese native speakers make when they themselves learn English (and possibly other European languages). As a local power, soon to be superpower, the Chinese language, alphabet and culture has dramatically influenced its neighbours and it seems to me that Chinese is a good first Far-East Asian language to learn, especially if one seeks to learn Japanese or Korean.