Directionality support

Support for different writing directions (left-to-right, right-to-left) is an important aspect for multilingual software. Since MediaWiki 1.18, the support has been considerably improved.

This page gives some relevant information, and guidelines for developers and wiki administrators if you want to make a feature or extension compatible with right-to-left interface and different directionality in general.

ResourceLoader
Thanks to ResourceLoader including CSSJanus, CSS is automatically flipped to right-to-left when the user language is RTL. This is default since 1.18 (in previous versions it was dependent on the wiki content language). There is relatively little CSS that must depend on the content language.
 * Attention: when using ResourceLoader's debug mode, the CSS may be inconsistently rendered compared to the production mode. As a temporary work-around test RTL interface with debug off. (Issue being tracked as bug 27025.)

What kind of languages?

 * $wgLang - user language (and the respective direction)
 * $wgContLang - site content language (and respective direction) -> here LTR
 * Title->getPageLanguage - page content language (and respective direction) -> by default the same as site content language
 * There are some default exceptions: see below
 * It can be set in extensions by the hook PageContentLanguage
 * This sets the TOC (and stuff like grammar, although not really relevant mostly) to that language. To do that only, use parserOptions->setTargetLanguage
 * In extensions that should remain compatible with 1.17, you can use wfUILang, however this breaks compatibility with 1.16 and below. This returns $wgLang in 1.18 and higher but $wgContLang in 1.17 (by default).

What should follow which kind?

 * Special pages follow user language by default.
 * Source code should be English (CSS/JS pages are set to English by default).
 * Pages with translated subpages should follow the language of the translation (e.g. "Page" is wgContLang, "Page/he" is "he"). This is by default for the MediaWiki namespace.
 * Input boxes and textareas follow content direction by default.
 * When viewing a diff, it follows the user language, but the actual diff text is in the page content language.

What should I add in the source code?

 * Anything on special pages that is different from the user language should have a tag.
 * Example: SpecialRecentchanges->setTopText which adds the "recentchangestext" message in the wiki content language.
 * Anything on custom namespaces that contains text different from the default wiki content should also have a div tag.
 * When the diff text is not the same as the page content language, use $diff->setDiffLang( 'code' ); where $diff is a DifferenceEngine object.

lang/dir attributes
You should use HTML: but sometimes that is difficult in dynamically generated elements and you can fall back on CSS:

Wiki content
You can use mw-content-ltr and mw-content-rtl classes to make use of edit section links and ul/ol elements based on that direction.
 * E.g.

This is of course used for the page content of articles.
 * On file pages, it is only added around the actual text while the rest of the file page is custom UI (file history, ...).
 * On category pages it is added as on regular pages, but the text in the user language is embedded in a div with lang & dir attributes according to the user language.
 * On special pages it is not added because these are largely in the user language. A piece of text on special pages can be in the content language, which should then be embedded in these classes. It is for example used in the CodeReview extension for the code summary and code comments, which can contain ul/ol elements (* and # wikitext) in the wiki content language and in principle not the user language obviously.

Adding UI stuff within these classes should not be done. For example, the category links and FlaggedRevs review form are inside #bodyContent but outside .mw-content-ltr/rtl.

A new feature is that these classes can be used within these classes. For example, .mw-content-rtl can be used in an LTR (mw-content-ltr) article. This is useful for multilingual pages. Prior to this, local MediaWiki:Common.css hacks had to be used to make e.g. bullet lists show correctly in  text. You should therefore use  instead of  when editing wiki pages.

See also Manual:Interface/IDs and classes.

body classes
The body tag has an ltr or rtl class by default, which follows the user language. You shouldn't use that, because CSS is automatically flipped based on the user language. The body tag also features a new (1.18) sitedir-ltr and sitedir-rtl</tt> class which follows $wgContLang->getDir</tt>. If there is need for it, a "userlang-ltr/rtl" and/or "pagelang-ltr/rtl" could be added as well.

float/text-align
You can use $wgLang->alignStart</tt>, $wgContLang->alignStart</tt>, $wgLang->alignEnd</tt>, $wgContLang->alignEnd</tt> which gives the appropriate 'right' or 'left' values.

You can use CSS float</tt> or text-align</tt> of which the value (left/right) is flipped for RTL interface languages.

Examples
Special:AllMessages is a special page, which means it follows the user language direction. However, the actual messages should follow the direction of the selected language. The cells in the table containing the content of the messages, have a lang & dir attribute.
 * Special:Allmessages = English, LTR
 * Special:Allmessages/he = Hebrew, RTL

The Translate extension, making use of the PageContentLanguage hook, sets the right language (and consequently direction) for translated pages, see e.g. Wikimedia_maintenance_notice/ar which benefits from the mw-content-rtl class on an LTR wiki.

Local wikis
MediaWiki:Common.css pages and relatives are also flipped. This means that you have to add @noflip where needed.

See bug 31923: site CSS should not be flipped by default

Issues

 * Most major features and extensions have now improved support, see translatewiki:Project:Better directionality and i18n development for that.
 * Known remaining issues:
 * External link icons are flipped according to user language direction (minor issue)
 * LiquidThreads: Use contlang instead of pagelang, otherwise special page returns userlang
 * WikiLove: kittens aligned left ( Blabla.jpg ) should be aligned right in RTL wikis (minor issue)
 * OmegaWiki: needs general overhaul (Expression and DefinedMeaning pages should follow user direction) + 23035
 * Search bugs: https://bugzilla.wikimedia.org/buglist.cgi?quicksearch=rtl