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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 I immediately thought of a point and click game that does something similar. In A Date in the Park, you play a guy who moved to Portugal, but doesn't speak the language. He goes to the park to meet up with the woman he has a date with.
There are several key moments where the language barrier ultimately works against you. I first watched a let's play of the game, where several viewers translated what was being said. Really changes the mood of a seemingly cozy game.