Each article on English Wikipedia (including its main homepage) displays a list of links in its left sidebar, that lead to the same article on other language versions of Wikipedia. These language links normally display in each link's particular language. For example, the link to the Chinese Wikipedia's version of an article will read "中国的", instead of "Chinese".
SidebarTranslate causes the language list to show up in English. It also provides popout Google Translate links for each, which allows you to go directly to an English-translated version of that foreign language wiki's article. Languages not fully supported by Google Translate have their links appear in orange rather than blue.
SidebarTranslate is available on Wikipedia as a "gadget" that I contributed to, and eventually re-wrote. Registered and logged-in Wikipedia users can enable it through Wikipedia's user preferences. I'm providing it here so that non-registered users, or users who are not logged in, can use it too. See here for SidebarTranslate's Wikipedia page.