Not the OP, but for iOS developers I've asked: "What's one thing you would change about Objective-C (this was pre-Swift), and what's one thing you would change about Cocoa [the framework that iOS apps are developed with]?" It's the kind of question that there are a lot of good answers to, any experienced iOS developer will be able to answer off-the-cuff, and it gives a good sense of the kind of work that they've done and how (plus it gets across if they know the difference between a language and a framework, which is useful).