Based on a reading of the show's script, it would appear that the phrase wasn't translated by the UT, nor do any of the crew recognise the phrase, despite many of them having had close contact with the Klingons on a large number of occasions. At the very least, this would indicate that Gowron's use of the phrase (perhaps his accent, a regional expression or the sheer formality of his speech?) are highly unusual in some way.
Whether this was intentional on the part of Gowron (perhaps because his use of an expression that is traditional, rather than an actual sentence with words that can be transliterated) isn't really clear. My personal guess is that the UT simply recognises when the speaker doesn't wish to be translated:
GOWRON: We shall see. (in Klingon) CHEGH-chew jaj-VAM jaj-KAK! (It is a good day to die!)
Gowron cuts off the transmission. All eyes turn to Worf.
WORF: (translating) He said, "Today is a good day to die."