All symbols were in Smith's vocabulary but he had trouble believing that he had heard rightly. (3.78)
We've all been here, haven't we? The words make sense but the meaning just won't click, no matter how hard you stare at the person speaking to you. Here we see Mike struggle with one of the barriers of language, the meanings beyond what the words literally signify.
"Here. I'll read it, then you put your thumb print in the square and I'll witness it. 'I, the undersigned, Valentine Michael Smith, sometimes known as the Man from Mars, do grant and assign to Peerless Features, Limited, all and exclusive rights in my true-fact story to be titled I was a Prisoner on Mars in exchange for—" (3.102)
Heinlein throws in a pithy commentary on how the tiniest change of language can completely alter a life's story. Just how different do you think the story I was a Pioneer on Mars would be? One word can make all the difference, can't it?
The woman's last speech had contained symbols new to him and those which were not new had been arranged in fashions not easily understood. But he was happy that the flavor had been suitable for communication between water brothers—although touched with something disturbing and terrifyingly pleasant. (4.51)
Language gets stickier for Mike and his understanding of Earth. Unlike the quote above, Mike doesn't understand the definition of the words—what we word geeks call denotation—but he does get meaning from them all the same. In a sense, he feels meaning from them—wordy geeky term here is connotation. Of course, Martian or not, everyone will have different connotations for words in various situations and circumstances.
"Yes, Jubal. You—" Smith stopped, looked embarrassed. "I again have not words. I will read and read and read, until I find words. Then I will teach my brother" (12.214).
Sometimes we have ideas that we just can't put into words. In our minds, they are clear as day, but when we try to express them, our tongue mangles and destroys it all. That's where reading comes in. By emerging ourselves in the words, thoughts, and ideas of others, we can also find the words to express our own thoughts. It's the moment when you snap your fingers and say, "That's the word I'm looking for!" Mike's reading is his attempt for that moment.
[I]t was not possible to separate in the Martian tongue the human concepts: "religion," "philosophy," and "science"—and, since Mike thought in Martian, it was not possible for him to tell them apart. (14.104)
People use words like labels on boxes that need to be categorized and stored. People label themselves, the world around them, and their ideas all the time: "I'm a scientist," "That's an arachnid, not an insect," and "Michael Bay films are schlock." Since Mike was born and raised a Martian, it's like the contents of every box were dumped out, reorganized, and then relabeled before being put away.
"You need to think in Martian to grok the word 'grok.'" (21.62)
Okay, so you need to think in Martian to grok the word grok, but one can't grok how to think in Martian until they understand the word grok—it is, after all, the central tenet of Martian thought. (Still with us?) So, how do the others learn Martian later in the book? Ah, the conundrums of language and thought.
"Mike thinks in Martian—and this gives him a different 'map.' […] Language itself shapes a man's basic ideas." (21.66-67)
In fancy speak, Mahmoud's idea is known as linguistic determinism. We all know that language affects thought through communication, but a linguistic determinist would argue that language determines thought and how we see the world around us. For example, if Eskimos have forty words for snow, then snow will be more important to an Eskimo than to someone who comes from a place with only three words for snow. In this example, Mike is the Eskimo.
"Mmm, one does have to learn to look at art. But it's up to the artist to use language that can be understood." (30.71)
Art is a language. We learn to read a piece of art, such as Jubal's sculptures, by reading other, similar types of art. For Jubal, it's a two-way street though. The artist communicates in the language of his medium—different mediums like paintings, literature, and music will have different languages—and in a way, the recipient can follow.
"That's the odd part. It's not really a church."
"What is it?"
"Uh, primarily a language school." (30.153-55)
The Church of All Worlds teaches its philosophy by teaching its own unique language. In a way, all social institutions are language schools. The NRA, the Writer's Guild, the Twilight fan club, they all teach a unique language and members must learn it to properly join.
Nevertheless Eskimos were invariably described as the happiest people on Earth. Any unhappiness they suffered was not through jealousy; they didn't have a word for it (33.94).
Here, Jubal proposes linguistic determinism just like Mahmoud did earlier. But consider this: when they first learned the word jealousy, did Eskimos start having the emotion right then and there? Or did they recognize it as an emotion they'd felt all along but just lacked a way to describe it? Hmm, we wonder…?