Disclose mental health issue during PhD during an interview?
I would suggest avoiding specifics unless pressed. "Personal reasons" should be enough for most. But the question may not even come up. There are enough reasons to miss "deadlines" in any sort of research program that most people won't give it much weight, unless very extended. Of course we like to finish things in a timely way, but the very process of research is inherently uncertain. 

You could even avoid the personal by citing "unforeseen obstacles" as your first response. It is fair to say "Personal" as the second version and "Health" as the third. Anyone pressing beyond that is being unfair as long as you assure them that the situation is in the past and not recurring.