Fixed spelling in lang.php

Changed "Erweitertet" to "Erweitert" since the word "Erweitertet" does not exist, see: http://www.duden.de/suchen/dudenonline?s=erweitertet

Also, see leo.org translation for "advanced" to german: http://dict.leo.org/ende/index_de.html#/search=advanced&searchLoc=0&resultOrder=basic&multiwordShowSingle=on