Answer:
Idioms don't translate well to other languages because it might not make sense when translated, or there are idioms in that language. Also, Idioms "do not translate" in the sense that different languages do not express the same meaning with the same idiom.
Hope this helps!
It allows the reader to see how the character came to be who he or she is currently, it creates realism in the text, it creates tension between the narrator and the character, etc.
