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?
Also not a book, but at the end of Godzilla Minus One, it really takes the surprise and emotion out of it if you know what Schleudersitz means. :/