It's not just how to make a robot explain itself, its decisions. Because even in plain good and old english there are level of understandings. Heck, even explaining some algorithms to someone IT-related isn't easy, when it comes from another field of knowledge.

When the AI(robot) decision is similar to a human daily thought, the difficulty is low. But even humans do things that they can't explain, and come with a later explanation that doesn't fulfill what just happened. When spurious correlations are found by AI, translating it to human understanding (not just language) will be very hard.