Interwiki links or interlanguage links are links to an article on the same topic in another language version of Wikipedia. These special links display in the left-hand sidebar under Languages, as first mentioned in Chapter 3 ("The Omnipresent Sidebar" on The Omnipresent Sidebar). These links to other-language Wikipedias appear under the native spelling of the language (such as Français) and are ordered by the two- or three-letter code for that language (such as fr or ja). Clicking the link takes you to the appropriate article in the other-language Wikipedia.
Any page, not just articles, can be interwikied. For instance, if you have a user page in Russian as well as English, you can add an interwiki link to the Russian version of your page on your English-language user page, and vice versa. Many help and community pages exist in multiple languages and are linked to one another in this way. These links can be very helpful if you want to find an equivalent project or policy in another language; for instance, if you want to find spoken articles in German, simply go to the English Spoken Wikipedia project, which has an interwiki link to the German-language Wikipedia page, WikiProjekt Gesprochene Wikipedia.
Editors must add links to other languages, article by article, for them to show up. The links are created using special language codes. These codes are mostly two letters (a few are three letters) and are based on the international standard [[ISO 639]], which catalogs languages. If no ISO code exists, a special code is developed and used; for instance, the Simple English Wikipedia (a Wikipedia written in simpler English) uses the prefix simple (according to [[Wikipedia:Multilingual coordination]]). These prefixes also appear in the Wikipedia URL for each edition: So http://en.wikipedia.org/ is the English-language Wikipedia. A table of all existing Wikipedia languages with their corresponding code may be found on the Meta site at [[meta:List of Wikipedias]]. These codes are also used informally on the projects to refer to the various language Wikipedias; you may see en:WP or enWP used to mean the English-language Wikipedia, de:WP to mean the German-language Wikipedia, ru:WP to mean the Russian-language Wikipedia, and so on.
Once you have found the two articles you wish to link and know their respective title and language code, creating the links is simple. Edit one article, and scroll to the end of the text. Interwiki links are placed at the very bottom of the article, underneath all article text.
The link takes this form: [[language code:article name in native language]]. For instance, if you're working on the article [[Cat]] in English, and you want to link to the article [[Chat]] in French, you would add the link
[[fr:chat]]
at the end of the English-language article. After saving the page, a link with the text Français will show up on the left-hand sidebar; if you click it, you'll be taken to the French article at http://fr.wikipedia.org/wiki/Chat. Similarly, to link to the article in German you would type
[[de:Hauskatze]]
which will give you a link to http://de.wikipedia.org/wiki/Hauskatze under Deutsch in the sidebar.
By convention, interwiki links are placed below category tags on pages, each on its own line. The most popular arrangement for ordering interwiki links on a page is alphabetically by code.
To be complete, you should also go to the articles in the other languages to add an interwiki link back to the first article (for instance, the page [[Hauskatze]] should also link to the English [[Cat]]). When creating interwiki links, add a simple +en: or interwiki as an edit summary. You may also find more interwiki links to that article to add to the original article you were working on. Today bots do much of this missing interwiki linking automatically.
To link to another language page without having it display as an interwiki link, use the same syntax but place a colon in front of the language prefix, as if you were linking to a category name. Typing [[:fr:chat]] on an English-language Wikipedia page will display as a light-blue link just as you write it that links to http://fr.wikipedia.org/wiki/Chat, but the link doesn't appear on the left-hand sidebar. Some important general principles still apply: Prefer the internal link means don't use links to another-language Wikipedia to replace a redlink to an English article and Seek outside references means you shouldn't rely on another Wikipedia to source important facts in an article. Translation isn't by itself sufficient verification, and other Wikipedia pages—no matter the language—are not acceptable as sources.