why does the creature work so hard to learn their language? what does that reveal about his character

Simply, the creature wants friends. He wants to belong and to feel wanted. This shoes us that the creature has the same needs as any of the people. He feels and is sensitive. He is willing to learn the language to fulfill the communication he so desperately wants.

This also means that he possesses intelligence.

When Shelley wrote Frankenstein, her intent wasn't to portray him as a "mad" beast. Movies created that monster, not the author. Shelley's creature is intelligent, possesses feelings, and is affected by his role as an outcast. Not belonging and not being accepted hurts him. Her creature was a model for society to look at and morally wonder how they react and treat the people around them, specifically if they're different.