We have always wondered about it. Now there are a few answers. An elaborate (PDF) whitepaper by Huiping Iler explains most of the difficulties involved in search engine rankings outside English. Most of these sites have filters, radio buttons and other specifics which act as obstacles during the search process.
Iler has some very interesting facts to offer. He says 60% of searches are performed in languages other than English. English has approximately 500,000 words whereas French has approximately 300,000. Now the really surprising bit. All forms of Chinese combined characters amount to approximately 50,000. Now, search becomes difficult as the words become lesser.
The optimization tips for such sites include better focus on language specific character encoding sets asw well as domain and hosting issues for search engines so that the site is properly categorized.