Or rather, you're not doing the efforts that you'd have to do to find good books worth your time in lesser-known languages. It's obviously true that there aren't many famous classics (famous among us, anyway) written in those languages you name, or in many others. But a lack of fame and repute doesn't necessarily mean a lack of quality.

I'll give it a shot, though. In Turkish, the obvious name would be Orhan Pamuk, and I must admit I can't really name any others myself, but I'm sure there are some. In Japanese, The Tale of Genji by Murasaki Shikibu has become rather famous as one of the first novels in world literature, while modern authors like Yukio Mishima (I intend to read and review one of his novels these coming weeks) and of course Haruki Murakami definitely have their fair share of international acclaim. I'm not sure how fluent your modern Arabic is, but since you said you'd read Mahfouz in translation as well, it's probably safe to assume you'd read his other books or other Arabic novelists in translation as well. Same for modern Hebrew, with the likes of Amos Oz, David Grossman and others. I know shamefully little about Chinese literature, but it's safe to say there are many works worth reading there as well. And that's just a handful of languages, albeit mostly large ones. Camilla is right about the Scandinavian languages, and I would, unsurprisingly, add Dutch (Mulisch of course, and Claus, but perhaps even more Lanoye).