So let me try to clarify to make sure I am understanding your point. In your example, you used "lamb" and "pig", where pig is the closest equivalent word in the target language. Your suggestion is that when there is no word in the target language, that the original word be left untranslated? Therefore, you would not use "pig" or "a new word", but leave it as "lamb"? Therefore, the reader is required to look up what the word "lamb" means in the orginal language, or at least to be aware that here is a word that he doesn't know the full meaning of.

John