First you need to install the language xpi that you want (Just click the .xpi to install it).Download .xpi file for Firefox/Swiftfox 188.8.131.52 from here
After you have installed the xpi file, open up a new window/tab. In the addressbar copy and paste
Hit enter and you will see a list of stuff. Now copy and paste the following into the search/filter area
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.