I want to use Google translate API from the iPhone.
What I found is : http://code.google.com/p/gdata-objectivec-client/
but in the list I cannot see the translate API.
Where can I find the library?
The Google Language API is provided as a web service. For non-javascript environments, you should be using JSON to communicate with Google's servers.
There doesn't seem to be an Objective-C wrapper to this functionality (like the ones you posted), so you'll have to roll your own. Thankfully, JSON is pretty easy to use on the iPhone.