"modern GUI system like Adobe Flex"
And yet Flash is a dying platform. Think about that. Complexity doesn't mean better. Localization only affects visible text for popup lists. You get the original non-localized values in the callback.
Although in all honesty, I've yet to encounter a valid use case where I needed to embed URLs in the popup list to begin with.
You will do yourself a favor by leaving all those adobe flex preconceptions behind when getting used to NGUI. That experience is an anchor that's holding you back. Step back and ask yourself... "why do I need to encode anything in the popup list's data?"
The only reason for it is if you've got several choices with the same exact text -- which is a ridiculous thing to have.
P.S. You do realize that your popup list choices can simply be IDs themselves, right? For example, you can do the following:
...and in your localization, you can set them up with proper text string values. Then your callbacks will just receive the "ID001" etc of your selection.