TL;DR: The oldest mention of a talking dragon would be from Chinese mythology.
It's important to note that the modern concept of dragons is an amalgamation of two primary sources - the European "Classic" dragon myth, and the Chinese Dragon of Asian mythology. Both are tales of large, serpent-like creatures but that's about where the similarities end.
In classic European mythology, dating back to at least the 8th century, the dragon was seen as a large, hostile creature which did not speak or fly. These creatures were typically seen as the embodiment of evil, and may have had their origins in the Middle East (where both the Muslim Quran and the Christian Bible make mentions of evil manifesting as large serpents).
Going even further, however, we find that the Chinese dragon first appeared in Asian mythology some time between 5000-2000 BC. In contrast to the evil creatures found in Western culture, Eastern culture depicts dragons as benevolent and wise creatures who represent celestial power. Eastern dragons almost always talk, and in some traditional stories it was, in fact, dragons who first taught mankind to speak. Eastern dragons also flew, albeit without wings. As part of their celestial power, they "rode the winds".
Modern dragon depictions have, for the most part, kept the traditional two modes separate. The "classic" dragon is still depicted as a large, dinosaur like creature with wings. Over the centuries, however, the classic dragon has begun to take on aspects of its Eastern counterpart - intelligence, ancient power, and (sometimes) benevolence. Although much of this merger has been popularized by recent depictions, aspects of the Asian dragon can be seen in Western culture as far back as Roman battle standards from around 270 AD.
The Chinese dragon, on the other hand, has changed very little in over 5000 years. Numerous Yangshao clay figurines & statues from the Neolithic Period have been found which bear a remarkable resemblance to the modern Chinese dragon. Even in modern Asia, dragons are seen as wise, benevolent, ancient creatures who represent good luck & success. They are almost never seen as evil or hostile, however, unless provoked by extraordinary circumstances brought about by men.
So, to summarize, modern dragons like Smaug are about 80% classic European dragon (looks & intent) with maybe 20% Chinese dragon thrown in (speaks & possesses ancient knowledge).