Manual:Language

In MediaWiki, there are various kinds of languages:
 * The site content language ($wgContLang, based on $wgLanguageCode), which should generally stay the same as long as the wiki exists.
 * The site content language variant (Manual:$wgDefaultLanguageVariant) is the default variant when LanguageConverter is implemented for that language.
 * The user interface language ($wgLang), which can be changed in your preferences or with  in the URL, but also generally remains the same while using the wiki.
 * The user language variant which is the variant of the site content language variant, as preferred by the user. It can also be set through  in the URL (e.g. by selecting one of the tabs).
 * The page content language. This can be different for each page, even if the site and user language is the same. It is defined in getPageLanguage in Title.

All three of them are a Language object.

Page content language

 * Default value
 * $wgLang on special pages.
 * English for CSS and JS pages.
 * For MediaWiki namespace pages, the language depends on the subpage. For example, MediaWiki:Message/ar will be set to Arabic (ar), and MediaWiki:Message will be $wgContLang.
 * All other pages are $wgContLang by default.


 * Configuration
 * Extensions can change all other pages through the PageContentLanguage hook. The value for special pages, CSS, JS, and MediaWiki namespace pages cannot be overridden.


 * Examples
 * The Translate extension uses it for the page translation feature. See Project list/ar as a translation of Project list. The directionality of the page is thus correctly set to right-to-left for Arabic.


 * What does it define?
 * In SkinTemplate, it adds a  around the page text. The dir attribute sets the correct direction. The lang attribute will always be the root code, e.g. "de" even when "de-formal" is given.
 * For file pages, it is set in ImagePage.php, because there is a lot of HTML that is in the user language.
 * In Parser.php, it sets the table of contents (TOC), and stuff like grammar, although not really relevant mostly. To do that only, use parserOptions->setTargetLanguage.
 * The direction of the diff text (DifferenceEngine) is set to the page content language. In some cases this is not identical, in which case $diffEngineObject->setDiffLang( $code ) can be used.

Multiple languages on a single page is in theory not supported, but simple  tags can be used to mark text as being written in a different language. If the CSS class is used, the ul/ol lists and editsection will display nicely when the dir tag is opposite to the value of that of the page content language. Things defined in the parser, like TOC and grammar, however, will not change.