I don't know that I've ever considered this question before.
Smaug would have used the Common Speech (Westron) with Bilbo, since it was the only language the pre Elf-friend Bilbo would have known.
Smaug could easily have known some Elvish Speech too though, at least some Sindarin. Probably not Khuzdul, though.
__________________
Music alone proves the existence of God.
|