There is currently no central place where various freedesktop.org packages can be translated. You should browse package pages to look for indications about how to contribute translations for each project.
However, you will be able to translate some freedesktop.org packages in the following infrastructures:
- Translation Project (http://translationproject.org):
* gstreamer, xdg-user-dirs, XKeyboardConfig, unicode-han-translation, unicode-translation
- Transifex.net (http://www.transifex.net):
* fprintd, shared-mime-infos, cups-pk-helper
Note that to participate to these package translations, you have to be a member of a team in the freedesktop.org Transifex project.