I set ConfigItem CustomerPreferencesGroups###Language to Valid="0" Yet the box still displays on my customers' preference page. I removed the ConfigItem completely, also to no avail. Yet these are the only files in the entire OTRS system, including the DB, that seem to have the words "frontend language" in them.

I think you need to create a new theme to do that. It's a little tricky but if you follow the documentation at http://doc.otrs.org/2.2/en/html/c1784.html you'll be set. It was the only way I could figure out how to remove the language box from the customer login screen. -- Lars