Fetching translated strings, keeping in the philosophy of Zend framework
The two real questions are:


What are the advantages of having this in Zend_Registry?
What would you loose when using a helper function?


I believe the answers to these questions lead to this: use the helper function. This will:


save you a lot of typing,
highlight more importants parts of the code,
only add a tiny new layer of abstraction which is trivial to understand.


"Good design" is not about factories, registries, or whatever. It's about code that is easy to maintain and improve. The function is not going to make this any harder. If there's a translation error, the fix will stay the same: was the key correct? Is the stored translation correct? Still easy.