“If one just said the words Baruch ata Hashem Elokeinu Melech Haolam (e.g., of asher yatzar) and there is a goy who asks you a question, what should you do? (It happened to me one time but I had only said Baruch ata Hashem so I quickly ended off with lamdeini chukecha.)”
What about smiling and holding up your hand, as if to say “please wait,” just as you might do if you were in the middle of a conversation with another person?