Recently I was asked to change my friend's system language from Chinese to English. Changing the language in account was not the problem but the log-in screen language was a bit of a challenge.
sudo /System/Library/CoreServices/Language\ Chooser.app/Contents/MacOS/Language\ Chooser
The same language window that comes up during installation will appear, select the new language, click the arrow. Log out. (You may get errors when the command finishes, despite this, things seem to work fine).
That should do it.