I just started Dungeon Crawler Carl. In it he fights a Spanish speaking boss. The Spanish is not translated (so far) and if you can read it, it really changes the mood and impact of the entire scene.
Are there any other books that hide significant plot points behind the use of foreign language?
Not a book, but the Norweigans at the start of The Thing literally give the whole plot away desperately trying to warn the Americans what the dog is
I watched that movie with some Swedes who were trying to be helpful and told us what the Norwegians were saying :|