If you look at the USE flags that the Entropy package maintainer used to build the Firefox and Thunderbird packages for the Entropy repository, you'll see that the Swedish language USE flag is turned off (USE="-sv_SE"): see the SL Packages page https://packages.sabayon.org/
So you need to install the packages using the Portage package manager. If you have not already done so, firstly, as root user in a Konsole/Terminal window, use the preparatory commands listed in the post Re: Atheros Wired Connection Problem. Then do the following:
Entropy packages are binary (i.e. compiled) packages, so you can't as it stands because there is no Firefox or Thunderbird package in the Entropy repository that was built with the Swedish linguas USE flag set. You could try filing a request (click on 'Bugs' at the top of the page) for the Entropy package maintainer to rebuild Firefox and Thunderbird with the sv_SE linguas USE flag set in addition. If he does that then, once he puts the rebuilt package in the Entropy repository, you would get it next time you use equo update && equo upgrade.
Ok thanks, I have installed the language pack from mozillas ftp for Firefox now and it is Swedish now but the Thunderbird language pack are just for Thunderbird 15.01 and I have Thunderbird 15.0 so I must keep looking for it for 15.0