I didn't realize it searched 5 sites, either. After all, at the top of the metadata page it says "calibre can find metadata for your books from two locations: Google Books and isbndb.com."
One of the plug-ins, for Douban books, is apparently for books in Chinese, as I found when I enabled it. I disabled it after finding this out as I can not read, write or speak the language/s.
Holly