http://www.weiwei-tv.com/scroll.php
As the video shows, I go to language->installed->administration and change the default language to en-US, although the message is in English, the menu items won't change back to English.
Then I clear Joomla's cache, change to a new browser, although the login page is in English, the back-end menu items are still not. I just began to manage this website which is built by other and I never had any trouble switching languages on my own sites.
What is wrong?