Manual:$wgLanguageCode/en

Details
The site language code is usually set automatically with the installer. List of languages codes can be found at. The code should be in lower case and use hyphen as separator. Examples:,  ,  ,

This specifies which language your page content is in, and also provides the default language for your wiki's interface. While users can switch the language of interface messages in their preferences, anonymous visitors will see the wiki's interface in the content language. $1 provides a way to auto-detect the language of anonymous users and let them choose their interface language without logging in.

Change existing users' language settings
You can change existing users' language settings with the maintenance script. They will still be able to change their settings back unless you disable that option (see below).

Disable user selection of language
If you want to have your wiki in a certain language and not allow users to change this setting, there is a new variable in 1.16 and above, :