Where, in chapter 12 of Lord Of The Flies, is the use of diction and syntax most apparent?

Diction refers to the vocabulary someone uses in speech or writing or it may refer to the manner in which one pronounces words. Obviously, the more educated the person is, the more complex or sophisticated his diction should be. Such a person would also be expected to articulate such vocabulary clearly. Syntax refers to the correctness of grammatical conventions the speaker or writer employs in order to present well-formed (grammatically correct) sentences. 
Furthermore, the type of diction and syntax used will differ in formal and informal situations and will also vary from person to person, according to his or her background, age, status, and so forth. In chapter twelve, there is an obvious contrast between the diction and syntax used by the narrator and that of the boys in conversation with each other. Ralph, for example, uses conventions which reflect his middle class upbringing and private schooling. This is apparent in his thinking:

“No. They’re not as bad as that. It was an accident.”

In this example, Ralph clearly expresses his thoughts in a grammatically correct manner - the language used is, one could, say semi-formal and evidence of a grammar-school upbringing. He continues using the correct conventions even when he speaks to Samneric:

“You two aren’t painted. How can you—? If it were light—”

The use of 'were' is significant in this example. Ralph uses the subjunctive mood, which is correct. One with a poorer education would, more than likely, have used 'was.' A further example is Ralph's statement that: “I only wanted to keep up a fire!” This also indicates sophisticated and educated English syntax. A more informal expression could be, "I only wanted to keep a fire burning!"
The diction and syntax used by the captain are also typical British grammar-school conventions:

“I should have thought that a pack of British boys—you’re all British, aren’t you?—would have been able to put up a better show than that—I mean—”
“I know. Jolly good show. Like the Coral Island.”

The use of 'should' as an alternative to 'would' and the phrase 'Jolly good show' identifies him as typically British. The Cambridge English Dictionary identifies the word 'jolly', for example, as mainly used in the United Kingdom.