Apparently there's something wrong with the zimlets on
http://gallery.zimbra.com
Because now I tried to deploy the same zimlet (i.e. com_zimbra_gtranslator.zip) from /opt/zimbra/zimlets-extra and that's working !
(I just discovered that most if not all of the zimlets can also be found in that folder)
Also the deployment of zimlets through the zimbraAdmin ui is working now.