Authors who use foreign language quotes in their books, but don't ever tell you what it means, so you may have lost a key inuendo (?), or clue about that character or the plot, or missed a snappy retort if the quote is used during an argument between two characters. Yeah, later I eventually look up what it means, but I don't want to stop in the middle of what I am reading to do that.
Yeah!!!
To this DAY I cannot read Elvish!!