Manual:$wgLanguageCode

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

NOTE: 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. (Beginning with MediaWiki 1.10, running rebuildMessages.php may no longer be required).

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.