One of the theories of the universal translator from TOS that most of the modern show's writers seem to have forgotten is that the UT, in addition to analyzing language patterns, matches the patterns with known concepts interpreted from a brain wave scan of the person speaking (see the TOS episode "Metamorphosis"). For example, if a person says "cat," his brain wave creates a unique pattern, different than when he says "dog." If someone speaking Spanish were to say "gato," their brain pattern would match the brain wave pattern of the person saying "cat," and therefore the computer would translate "gato" as cat, and similarly "perro" as dog. Therefore, if the translator doesn't translate a word, it must be because the speaker knew he was being translated but wanted to be heard in his original language for poignency, and the UT read this brain wave pattern and didn't translate the word! Obviously, this happens A LOT throughout Trek.