In the film's official novelisation, we get a slightly different version of events. The UT is producing some pretty weird results and they're forced to rely on the paper books from the ship's library. On top of that, they get very lucky in that the border guard is a) Very drunk b) Hugely amused by their poor attempt at Klingon and c) Mistakes them for ale smugglers.
"Sir," Uhura said suddenly. Spock turned at the note of panic in her tone. "The universal translator - I was just programming it for Klingonese, but it's not responding. It's coming out all garbled."
Spock moved swiftly to the communications console and swiftly ran a diagnostics; both he and Uhura read the results and gave each other a knowing look. Even if Enterprise ran cloaked into Klingon space, she still risked detection by specially equipped listening posts inside the Klingon border. And without the translator to aid them, the risk of attack escalated enormously. The conspirators were at work again. At the moment there was nothing to be done.
Resolutely Spock returned to the command console. "Mr. Chekov, set a course for Rura Penthe." At the helm Chekov swiveled to face the Vulcan. "Mr. Spock, Rura Penthe is deep within Klingon territory. If we're discovered-was "Quite right, Mr. Chekov," Spock allowed smoothly before the helmsman could finish. "If, as I suspect, all of our computer data banks have been stripped of all information on the Klingon language, then we will have to rely on the archive library. What is now required is a feat of linguistic legerdemain - and a degree of intrepidity -before the captain and Dr. McCoy freeze to death."
Mortagh Outpost Three was understaffed, underfunded, decaying-in short, one of the least sought-after assignments in the Empire, which suited former gunner Kesla perfectly. His one taste of battle-years ago, in a pointless skirmish between his ship, Beria, and a Romulan vessel whose name he had never learned-had resulted in the death of his captain, most of his crewmates, and his closest friend. Kesla had escaped with severe injuries and a determination to leave the life of a warrior behind, Klingon honor be damned. He knew his job was looked down upon by respectable Klingons; he knew that he fit the stereotype of the spineless, drunken, dozing outpost sentinel. He did not care. He welcomed the isolation and the lack of responsibility - in all his years at Mortagh, he had never heard so much as a rumor of a Federation ship attempting to steal across the border, though those who worked the outposts near the Romulan border told a far different story.
There the outposts were properly equipped with the latest detection devices. Mortagh Three creaked with age - a metaphor for the decaying Klingon government.
Kesla and his peers were forced to perform their jobs with scanners that had been outdated a century ago, without proper visual displays for viewing passing ships. They were even without weapons; their only threat lay in informing the Empire of the offending vessel, then hoping the border patrol managed to destroy the right ship. Smugglers favored the route; the sentinels had long ago given up trying to stem the tide.
Kesla did not welcome the boredom, but there were ways of diluting it-for example, with the fiery Catullan liquor he'd consumed that evening.
Access to black market wares was one of the occupational benefits of working the border. The Catullan liquor, qrokhang, had taken the edge off Kesla's restlessness, if not his boredom, and eased him into a pleasant numbness. Now he sagged against his scanner console, skirting the edge of sleep, images from the long-ago battle clouding.his euphoria. . The scanner beeped a warning.
Kesla straightened and tried to shake himself awake. A feeble light on the aging screen blinked at him, indicating a cloaked vessel.
Klingon ships were forbidden by law to remain cloaked while passing the outpost. The smugglers were well aware of that fact and knew it was safer for an unregistered vessel to pass by openly-at least that way, the alarm wouldn't wake those working the outpost.
They did not know that the halfhearted adjustments made to the ancient scanners to detect the more recent innovation worked only half the time.
Kesla frowned, displeased that his nap had been disturbed. This smuggler was apparently new to the business. He punched a toggle on his console.
"This is Sentinel Kesla of Mortagh Outpost Three. What ship is that? Identify yourselves" A pause followed - one so long that the nearby operator, Genrah, yielded to curiosity and wandered over to listen next to Kesla. Kesla shifted to allow him room. Mortagh Three's unspoken rule insisted that the sentinels share any opportunities to alleviate boredom. Both of them frowned at the console.
The reply came in a feminine voice laced with static. "We am thy freighter . . . Ursva, six weeks out of Kronos. Over." Kesla shared a surprised look with Genrah. Female Klingon smugglers were rare indeed, but if this one was Klingon, Kesla was grandsire to an Earther.
"Whither are you bound?" Kesla asked gruffly responding in the odd archaic dialect the female had used. It was no small feat to keep the amusement from his voice. This smuggler was probably Rigellian or Catullan, in which case Kesla owed her a debt of gratitude. But if she knew so little of smugglers' protocol in the Klingon Empire, she would probably never reach her destination. Genrah and Kesla waited, drowsy but mesmerized, through another delay. "We is condemning food . . . things and . . . supplies to Rura Penthe. Over."
At this point, Kesla and Genrah both laughed aloud at the ridiculousness of the lie. The smuggler was so inept that Kesla pitied her. He waved at Genrah to let her pass. If she was hauling Catullan liquor, Kesla wished her good fortune in her task. She would need it. To let her know he knew full well who she was and what she was doing, Kesla uttered a phrase which, in smugglers' code, wished her luck in avoiding border officials.
Uhura clutched the edges of her console and stared at the communications board. The last comment by the Klingon had left the crew entirely mystified, and if she failed to respond properly... Nearby, Chekov and Spock glanced up from the antique paper dictionaries that littered the consoles. The past few minutes had been harrowing; had it not been for Spock's ability to rapidly scan the archaic alphabetized glossaries and to perfectly recall the few sentences in Klingon he had ever heard, Enterprise would probably have been nothing more than space flotsam by now. But, according to Spock, the Klingon had just told her not to catch any bugs - then dissolved into raucous laughter.
Uhura glanced for guidance at the Vulcan, whose expression, though impassive, nonetheless conveyed his total bewilderment at the idiom. The raucous laughter faded; Spock gave Uhura a nod. She turned back to the board and dial her best imitation of Klingon laughter; the effort made her throat sore.
The Klingon closed the channel. Uhura waited tensely for a few seconds until the realization settled in: the communication was over. They had, for reasons she could not fathom, been granted clearance.