XZ923 said:
I know exactly what episode you're talking about (it's the one where they get stuck in the repeating-time loop colliding with Kelsey Grammer's ship). I think the simplest explanation is simply that Worf is fluent in both Klingon and English (which is apparently the universal language of the Federation) and he's simply using a Klingon phrase. It's similar to the way a bilingual person will occasionally insert words or phrases from their native tongue into a sentence in an adopted one.
Q: What do you call a universal translator that doesn't translate all the Klingonese it hears into something you can understand?
A: Broken.
If I am wearing a device which translates my words for everyone while suppressing the sounds from my mouth (and it does - the away teams regularly encounter life that does not have universal translators, yet understands them), and translates all the spoken words I can hear into a language I can understand (which it does, because the away teams in turn can understand those aliens), then the moment I hear anything non-English at all, I will immediately replace my universal translator.
If a Klingon used Klingon words for effect, and they were not translated, I would freak out. How would my personal universal translator know that the Klingon didn't want those words translated? What else would it decide I don't need translated because the word was being used for effect? Imagine:
(Two bomb technicians, working on an explosive device)
"That's it... that's it... ok, now cut the [unintelligible sound] wire."
"The... what?"
"The [unintelligible sound] wire. Cut it. We have ten seconds."
"I didn't quite catch that."
"[unintelligible sound]. [UNINTELLIGIBLE SOUND]!" *gesturing wildly at the bomb*
"Uh..." *looking panicked* "Did you say Red?"
"NO! I SAID [UN - IN - TELL - I - GE - BELL S-OOOOOOO-UUUUUUU-NNNNN-DDDDDDDD] YOU IDIOT! CUT IT OR WE ALL DI-"
*boom*