A very well written elaborate (PDF) whitepaper by Huiping Iler explains in great detail what's involved, and most of the difficulties, with search engine rankings outside English. Consider that users are faced with filters, radio buttons and other obstacles during the search process, and you can picture what must be done to establish visibility with your target audience.
Consider the facts pointed out by Iler, such as 60% of searches are performed in languages other than English, English has approximately 500,000 words, compared with French having approximately 300,000 and all forms of Chinese combined characters amount to approximately 50,000. One can surmise the increasing difficulty of optimization with the decrease in available words.
Although the optimization tips are pretty straight forward, special attention is given to detail such as language specific character encoding sets, and examples of what it looks like for users when they're incorrect. Other good advice includes hosting and top-level domain issues for search engines to get your site properly categorized and served in the region of interest.