Change language in Firefox and Swiftfox web browsers

First you need to install the language xpi that you want (Just click the .xpi to install it).Download .xpi file for Firefox/Swiftfox 2.0.0.1 from here

After you have installed the xpi file, open up a new window/tab. In the addressbar copy and paste

about:config

Hit enter and you will see a list of stuff. Now copy and paste the following into the search/filter area

general.useragent.locale

Now you will se only one entry, doubleclick on it and add the code for your language (the same as the xpi file).
So if you installed the sv-SE.xpi you enter sv-SE.

After this, restart Firefox/Swiftfox and you have your preffered language.