Untranslatable Words?

We, translators and interpreters, have the challenging but fascinating job of expressing the same idea in different languages. There's a whole mental process involved, which in the case of interpreters requires extra effort as it should be performed fast.  

Throughout my career I've encountered words that imply the use of paraphrasing techniques in the target language due to the lack of a word with exactly the same denotation (http://www.merriam-webster.com/thesaurus/denotation) and connotation (http://www.merriam-webster.com/dictionary/connotation).

One of the words that poses difficulty to me is insightful, which means showing a very good understanding of a person or a complicated situation, Macmillan dictionary. We can collocate this adjective with  parent/book/behaviour/article/commentary. A suitable word in Spanish would be perceptivo, which means having the capacity to receive sensations through the senses. However, does this mean that you show a good understanding of a person or complicated situation. I wonder... Other adjectives in Spanish that would be used to express this idea would be revelador, intuitivo. Thus, un comentario revelador would be one that offers good understanding of a person or complicated situation. I'm not sure the whole concept is depicted in this way. 

Can you think of adjectives in Spanish that reflect the whole concept imbedded in the term insightful?

Have you had trouble finding a word in the target language that really expresses that idea conferred in the source language? Please share! I'd like to know.