One of the quirks of Pirahã culture/language that stands out is the absence of "phatic" communication: "communication that primarily functions to maintain social and interpersonal channels, to recognize or stroke ... one's interlocutor," as Everett explains. The Pirahã language is free of expressions like hello, goodbye, how are you?, I'm sorry, you're welcome, and thank you—utterances that, as Everett puts it, "don't express or elicit new information about the world so much as they maintain goodwill and mutual respect." Rather than a verbal expression of appreciation for a favor or gift, "The expression of gratitude can come later, with a reciprocal gift, or some unexpected act of kindness..." Similarly, Everett says, "They have no words for I'm sorry. They can say, 'I was bad,' or some such, but do so rarely. The way to express penitence is not by words but by actions."
I've just finished reading Dan Everett's account of his life among the Pirahã people of the upper Amazon, called, Don't Sleep, There Are Snakes: Life and Language in the Amazon Jungle. Everett has spent practically his entire adult life among the Pirahã (pronounced pee-da-HAN), who are fascinating in many surprising ways that have forced linguists and psychologists to revisit some of their most central assumptions about universal human traits. (See this New Yorker profile of Everett, written by John Colapinto for an excellent summary.)