Can’t say for how Blizzard intended it, but I feel like we can safely use D&D as a reference, as it is likely that is from where Blizzard drew their inspirations.
Cross referencing with various editions of the game, a druid’s shapechange is essentially the druid taking the form of a known creature, typically a beast but sometimes even elementals or fey creatures (A Worgen, for a Warcraft example). The Druid attains all the physical characteristics of the creature, while maintaining the mental characteristics of their original form. Thus, a Druid in an animal form still maintains their ability to understand speech, read, think critically and recall information all the same as they would as their unchanged selves.
That is a pretty consistent interpretation across all editions of D&D, Polymorph is less consistent.
Ill use 5th edition Polymorph, as it is more like what we see in WoW.
“This spell transforms a creature with at least 1 hit point that you can see within range into a new form. An unwilling creature must make a Wisdom saving throw to avoid the effect (Can resist the magic). A Shapechanger automatically succeeds on this saving throw. (A shapechanger would be things like Werewolves, Doppelgangers, Mimics, so on.)”
Unlike a druid’s shapechange, a polymorphed creature adopts both the physical and mental characteristics of the new form. So if you are polymorphed into a sheep, you are as intelligent as a sheep. If you are a druid who shapechanged into a sheep, you are a very smart sheep.
This is pretty consistent with the way the two abilities are depicted in WoW. The interesting thing is, in WoW, Polymorph is mostly used defensively. A Mage using it to turn a threatening creature into something less threatening. In D&D, while the spell can be used that way, it is more often used to do the opposite. To turn an ally, or one’s self, into a significantly dangerous beast to vanquish their foes.