Cool app to switch your Language interface in Mozilla products. Rated 5 out of 5 stars
It supports both Firefox and Seamonkey and I must say it works as advertized (needs browser restart after changing a language).
It is needed tool to anyone wanting to use it's browser with many languages and on many platforms (i am on Solaris/Openindiana x86 for example) and have add-on that works as advertized no matter what platform you use.
Also guy making it is a nice fellow and he's work is open source and freely licensed, so be free to contribute to him and to show him your support! :P