Manual:$wgLanguageCode

Details
Site language code. Usually set automatically with the installer. List of languages codes can be found at language/Names.php. 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. Language selector extension provides way to autodetect the language of anonymous users and let them choose their interface language without logging in.

If you change this after installation, in MediaWiki 1.18 and above you can clear the message cache by running. Be prepared to update all messages you have customized in your wiki, move your main page and other broken things before and after all caches has been regenerated.

In MediaWiki 1.8 or older, if you change this after installation, you should run the maintenance/rebuildmessages.php script to rebuild the user interface messages (MediaWiki namespace). Otherwise, you will not see the interface in the new language, or a mix of the old and new languages. Note that running that script will override any custom interface messages you may have created.

$wgContLanguageCode gives the same value as $wgLanguageCode, but is deprecated in favor of $wgLanguageCode.

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

php userOptions.php language --old en --new cs

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, $wgHiddenPrefs: