I'm pretty certain that there are works of fiction purely in a fictional language. For instance I'd be surprised if some uber-geek hasn't written some fan fiction entirely in Klingon. I also know there are many stories where the characters in the story learn and become fluent in a foreign/fictional language as the plot progresses. Finally I know there are stories that use a smattering of a foreign/fictional language in their text/exposition.
That being said what is the first science-fiction or fantasy work that fully committed to the leap? Have any stories started out in English for example and as the character starts to learn the native language the exposition gradually changes to be mostly or completely in that language? Something like chapter 1 is entirely English, and by the time we reach chapter 100+ it is >= 90% fictional language X? It occurs to me that this would be an interesting and maddening type of story to read.
While I would certainly accept an answer with a transition between two contemporary1 languages like English to Spanish, or Roman Latin to Ancient Greek, I would be especially interested in knowing the first instance an author attempted this with a fictional language that was made out of whole cloth for the story.
After some more poking around on this site I found this answer which mentions Feersum Endjinn. According to the linked Wikipedia article approximately 25% is written in basically a pidgin English. That type of language switch isn't on topic for this question, because with a basic to good knowledge of English you can clearly read the other language. For that reason a difference caused by a pidgin/creole/dialect isn't what I'm looking for. If someone wants to press the boundary between a creole vs a language though I'll give them the benefit of the doubt.
1: Languages commonly spoken at the same period in history.
I've edited based on a Meta discussion.