Thread:Project talk:Support desk/Invalid language code "nl"

After I added $wgLegalTitleChars, and $wgShowExceptionDetails MediaWiki throws me a error:

Invalid language code "nl"

Backtrace:


 * 1) 0 /var/www/wikifarm-mw1.19/languages/Language.php(149): Language::newFromCode('nl')
 * 2) 1 /var/www/wikifarm-mw1.19/includes/Setup.php(473): Language::factory('nl')
 * 3) 2 /var/www/wikifarm-mw1.19/includes/WebStart.php(157): require_once('/var/www/wikifa...')
 * 4) 3 /var/www/wikifarm-mw1.19/index.php(54): require('/var/www/wikifa...')
 * 5) 4 {main}

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What is causing this?