That would be so simple if just everybody wanted, and could learn English, wouldn't it ?
They can, and they will be rewarded with speaking a decent language

To be more precise there are two ways for you to add translations:
add the translated text to the respective po-file and then convert the po-file to a mo-file (done automatically in po-edit), then
1) in your running, localized system:
- put the mo-file into /usr/share/locale/<language, in your case "fr">/LC_MESSAGES/
2) in general and for addlocale to pick up new translations:
- update the rpm with the updated mo-file
- once the updated rpm is in our repo then the updated translation is available for
a) everyone: don't ask but instead patiently wait for Pinoc to rebuild and upload new language archives (every 1-2 months); after that run addlocale, or
b) individuals: reinstall the updated rpm on your localized system (which is the same as 1 above)
Don't know about a general place for the right-click menu. In fact, what you are looking for is called
Service Menus in KDE4, it's simply a .desktop file placed in /usr/share/kde4/services/ServiceMenus/ so you may have a look there. I don't do Gnome, so no idea how this works there, I think in Gnome they are called
Actions, maybe
this or
this helps.
regards,
-p.