The Horta is an interesting case study of the difficulties of translation. First, there is the mechanical hurdle—this creature has no speech organs. As Phillip Morrison indicated, in a diversely populated universe what we think of as speech can hardly be universal. We have to wonder, how quickly would we recognize intelligence and communication from totally alien life-forms? The seemingly omniscient UT simply is no match for the Horta (or the whales, or the mystery probe that came to find them).Spock makes an attempt to mind-meld with the creature, and has only limited success. But there was enough contact for the Horta to learn to write "NO KILL I" in the rock, thus overcoming the aural communication barrier. It's amazing that the creature could so quickly learn even a rudimentary version of written English, considering a written language would probably be as foreign to these creatures as a spoken one. (I also think it would have been interesting to see the Horta pick up some Vulcan script instead of English - or a mixture of the two.) Nevertheless, this is one of the most creative attempts at communication we've seen in Star Trek.
The syntax of the message demonstrates another challenge of translation. Kirk wonders, "'NO KILL I' What is that, a plea for us not to kill it or a promise that it won't kill us?" When we learn another language, our precision suffers. Even the Universal Translator would have this kind of learning curve. There must be times when a translation is vague enough to be confusing (which is why the TNG Technical Manual suggests that the UT cannot be used for diplomatic purposes without adequate exposure to the target language).
The communication gap is finally bridged by a full mind-meld. At first even this mind-to-mind communication produces only cryptic phrases. But as with the whales in Star Trek IV, Spock’s telepathic ability ultimately provides a level of understanding no conventional method could accomplish. What makes this possible? Well, we return to the same premise that makes a mind-reading UT at least plausible—namely that there really are universal thought patterns and an underlying structure to language that can be detected and understood.