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?
It's a dark and political story wearing the skin of a light hearted romp like a serial killer with their victim's face.
It takes a book or 2 for the mask to really slip (hence the hammyness of the first book or 2). It's an excellent story, though a little marmite like (you either love it or hate it).