- Is it just me, or does Mr. Wonka seem a few quarters short of a buck?
- I’m sorry, I don’t speak American.
"a few quarters short of a buck" suggests something incomplete or lacking. It is one the (jokey) euphemisms for being slow in thinking, or "stupid" which seems to be a taboo word nowadays.
So the first person is suggesting Mr Wonka might not be a particularly sharp minded guy, and the other person is essentially saying "I'm sorry. I don't want to talk about it" in a roundabout way which is polite but still sarcastic, probably offended by that suggestion. Since "American" in this sense is not an established word, they probably mean "Don't give that nonsense/bullshit".