I resolved this by copying the .properties files also to zimbraAdmin webapp
(cp *.properties /opt/zimbra/tomcat/webapps/zimbra/WEB-INF/classes/msgs/
cp *.properties /opt/zimbra/tomcat/webapps/zimbraAdmin/WEB-INF/classes/msgs/
cp ZsMsg*.properties /opt/zimbra/conf/msgs/).
I'll edit the wiki (
Translations - ZimbraWiki) to reflect this. I suspect not all .properties files need to be copied everywhere but I'm not going to experiment. Someone correct me if I'm wrong, this is what works for me.