What does sedaris mean when he writes that "understanding" another language is "a small step, nothing more, yet its rewards are intoxicating and deceptive" (?

From that quote, he stated that understanding a language eliminate the first barrier that people have understanding another culture.

But often time, people misinterpreted understanding a language as understanding the culture itself, which is why it is deceptive. There is no way for us to truly understand other culture unless we actually live among it.