How to change firefox to whatever language you like

Pretty much useless unless you know firefox so well that you know what the menu items are, then you can impress your friends choosing a weird language. I just know more than one languages :D, so I changed the language-pack from time to time.

Go to http://releases.mozilla.org/pub/mozilla.org/firefox/releases

Pick the right release

Go to the \xpi folder

pick the language-pack you want and click on the xpi to install.

then type about:config in the addressbar, find general.useragent.locale and edit it to the corresponding language code.