I'm especially surprised I never see trending in Arabic, Chinese, Korean or Japanese. Does Twitter intentionally hide them or do they just not Tweet much?
Probably is cultural. As a spaniard, my guess can be that people love gossip but the amazing thing is, every tv show and many ad agencies use twitter hashtags for their promotions. This didn't happened me at Berlin, for example, but maybe I'm wrong and it's just something cultural.
It is not exactly like that, I saw both chinese and russian trends on twitter, rember #дождь for example.
I guess people just tend to twit in English, even if it is not their primary language.
Chinese is hardly present on Twitter because of censorship in China and competition within China from Chinese-language microblogging services (which are very influential within China).