Make $wgDummyLanguageCodes an associative array so it maps to the correct codes;...

Make $wgDummyLanguageCodes an associative array so it maps to the correct codes; and use it in WikimediaIncubator to check whether the wiki exists (so e.g. Wx/sgs recognizes that the wiki corresponding to Wx/bat-smg exists). The extension is compatible with the old $wgDummyLanguageCodes array.