You can find lovely examples of this in The Lord of the Rings -- like the chapter "The Voice of Saruman" or how the language of the narrator modulates in describing Sam being tempted by the Ring versus his more down-to-earth thoughts. (IIRC. I don't have it handy to check.)
LoTR gets way more complicated than this, but you're right. There are very localized changes in language and dialogue throughout the book, and they're all intentional.
Note that the Rohirrim alliterate, for example (they're saxons!), and the elves don't.
