I think you all met with this problem. A new version of Zimbra came out and you had to update your translation. But how?
I also had this problem, investigated a few solutions and described one on the wiki in detail:
Translations - Zimbra :: Wiki
It uses translate-toolkit to convert .properties files into .po files, which are easy to update. Feel free to correct typos and clarify things! I hope this will be helpful.
Moderators: Could you please make this topic sticky? If you look at posts in this section, this is a way too common problem.