Fixing the language errors

[holyfive]

Hello,

I had to fix some issues to get phpScheduleIt working in German. Would you please take these changes into the official version?

In detail:

* In "lang/AvailableLanguages.php" change "du_de" to "de_de" and "du.de.php" to "de_de.php"

* Add "require_once('en_us.php')" to "lang/de_de.php" and to "lang/fi_fi.php"

Thanks in advance!

[Nick]

Will do. Thanks!