Zing! Locale Switcher provides a quick, easy, and reliable way to switch your browser between different locales that are available to it. First, download the appropriate language pack(s) for the locale(s) you wish to use the browser in, and once installed they will appear in Zing! Locale Switcher's dropdown locales menu. From there just click on the desired locale to switch to it.
For more information on the extension, please see:http://gooeysoftware.com/mozaddons/zinglocale/
For discussion, bug reports, support, etc. please see:http://forums.mozillazine.org/viewtopic.php?f=48&t=2820977