Neither geek nor nerd is an adequate translation. However, in modern use, both words may carry a shadow of the right connotations of obsessive interest and/or social ineptitude; see the geek and nerd pages for details. Think of the older, more pejorative senses of geek and you're on the right track. The British term anorak and the American term wonk are also close translations, and in more dickish corners of The Internet, autism-related terms like aspie get slung around in a similar manner. The closest troper-speak cognate would be "Loony Fan." In Japan, the term Otaku does not carry a positive meaning, at all. One of the first things most Japanese language classes often have to teach people is that calling yourself an Otaku in Japan is a very bad thing.