Manual:$wgContLang

Details

The content language object is the Language object associated with the wiki being viewed. It is one of two global language objects ($wgLang is the other). Language objects represent the language used for incidental text, and also has some character encoding functions and other locale stuff. The language is defined in $wgLanguageCode . See Manual:Language for more information.

If you are outputting text that might be included in an interface message, you may want to use $parser->getFunctionLang() instead of $wgContLang since this will localize to the user's interface language within interface messages, but localize to the wiki content language otherwise.
MediaWiki version:
1.32

Starting with MediaWiki 1.32, the content language is available as the ContentLanguage service (MediaWiki\MediaWikiServices::getContentLanguage) and usage of the service is recommended over using the global.

Often used Methods

This is not an exhaustive list, view class above to see a complete listing.

$wgContLang->getNsText ( $index )
Get a namespace value by key in the current language. Examples: $wgContLang->getNsText( NS_MEDIAWIKI ); returns "MediaWiki", $wgContLang->getNsText( NS_CATEGORY ) returns "Category".
$wgContLang->specialPage( $name )
Get a localized special page name by the English term. Example: $wgContLang->specialPage( 'upload' ) returns "Special:upload".
$wgContLang->convert( $text, $isTitle = false)
Not implemented. Convert text to different variants of a language.
$wgContLang->formatSize ( $size )
Format a size in bytes for output, using an appropriate unit (B, KB, MB or GB) according to the magnitude in question
$wgContLang->formatTimePeriod( $seconds )
Format a time in seconds for output, using an appropriate unit (seconds, minutes, hours or days) according to the magnitude in question