Have a look
here and
there.
You can request to create a new language team for Farsi in the Salix project, if you are eager to work at it yourself, but there can be hurdles as it seems that there is no UTF-8 encoded locale for Farsi in the glibc shipped with Slackware (only fa_IR, not fa_IR.utf8 as reported by "locale -a|grep fa") and I assume that Salix ships the same glibc.
But to answer more directly your question, localization of Xfce is not done by the Salix project but by the Xfce project and they have a
Persian translation team that you could also join. Generally a translation in a specific language is included in a software when it is at least 80% complete. This is not the case here so Farsi is not yet available for Xfce. When it will be included in some Xfce version all distributions that will ship that version will provide it.