Handles editing of a particular set of language entries.

• Uses the ManageSettings language file
• Uses the modify_language_entries sub-template
• Requires a valid language ID (lid) in the URL
• Saves primary settings if $_POST['save_main'] is set
• Handles editing of a single file and displays a warning if that file isn't writable
• Cleans the language cache and redirects back to the main editing screen for that language upon saving