Update de_DE.ini

Corrected and updated existing strings, translated some untranslated ones and gendered the German equivalent of "user", which didn't include female users.

lang/de_DE.ini

@@ -5,7 +5,7 @@ add_link = "Link hinzufügen"
 add_menu = "Menü hinzufügen"
 add_new_page = "Neue Seite hinzufügen"
 add_new_post = "Neuen Beitrag hinzufügen"
-add_source_link_optional = "Quelllink hinzufügen (optional)"
+add_source_link_optional = "Quell-Link hinzufügen (optional)"
 add_sub = "Unterseite hinzufügen"
 address_url = "Adresse (URL)"
 admin = "Administrator"