Manual:$wgLanguageCode

Details
Site language code. Should be one of the ./languages/messages/Messages(.*).php files.

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, i.e. the one specified by $wgLanguageCode.

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.

Hints
In 1.6.8: the string is case sensitive. The installer installs though small (for example "de"). This results into only a partial use of the language file. This is fixed if you use "De", which then uses the LanguageDe.php correctly.

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: