Intelligent can mean a number of things, so to simplify, lets limit it to capable of human speech. It is a common quality in movies and books to give dragons the power of human speech, but are such qualities given to any dragons in mythology?
Fafnir, in the Völsunga saga, is a dwarf that slew his father and stole a fabulous treasure, including the cursed ring Andvaranaut. Fafnir then turned into a dragon. Fafnirs brother Regin trains the young hero Sigurd, and eventually goes with him to slay Fafnir. As Fafnir is dying, he speaks to Sigurd, and tells him that his gold and Regin will kill him.
In another myth, we have the dragon Níðhöggr, who is gnawing on the roots of Yggdrasil, the world tree. Snorri's Edda as well as Grímnismál tells of the squirell Ratatosk, who runs up and down the three bearing words between Níðhöggr and the Eagle sitting in the tree's top, meaning that Níðhöggr must be capable of some sort of communication
In Slavic mythology dragons can speak, they command armies and rule over lands. They can assume human form and do not hoard gold for the most part. Some dragons are more bestial and have several heads, but some can speak.
I am sorry that I gave no examples, but I currently don't have the books for reference.
In Greek mythology most things that are similar to dragons are beasts. Hydra can't speak, I never heard that Scylla or Charybdis could speak. Basilisks can't speak. So no speaking reptiles that I know of.
Chinese dragons can clearly speak, such as Ao Guang. (He is more like a god, but Chinese dragons have a different position from European dragons.) They seem to gather treasures (Ao Guang was rich), but it seems that they are more interested in rarity, so it is more like a collection. In Journey to the West, Ao Guang gives Sun Wukong many magical items.
So they can speak, they can rule, they have a palace. European dragons almost look like barbarians next to them.
"Dragon" can be too broad. The answer will be dragon-dependent. Only two examples:
The dragon of St George appears be only a beast.
In the Grimm Brother's tale "The Devil and the Soldiers" a dragon offers to save three soldiers if they agree to work for him for a number of years (Wikipedia). I don't know how old is the original tale.