The one thing I like about CodeIgniter (and CakePHP) over Zend is that there is a defined set of principles to follow to get your website cracking straight away. Zend is extremely powerful (moreso than most other frameworks), but there is very little "hand holding" in leading you through the tutorials. Particularly after the release of 1.8, it's taking the documentation a little while to catch up.
I would highly recommend CodeIgniter, if only for their documentation. Most frameworks are offering similar outcomes, with varying promises on reliability, scalibility and security along with "pretty" URLs and such.
The CodeIgniter community is extremely friendly, and most importantly it's centralised. Everybody goes to the same place (although there are numerous third-party forums and doc sites out there), and everybody reads the same outcomes.
I've used CodeIgniter for developing UTF-8 supported web apps, but ended up hand building logic to determine the user's chosen language from the HTTP request information.
Hopefully that provides a little insight. Best of luck with your project!
Josh