Some of this assumes that they're always using it, which was never my impression. When Starfleet officers are talking to one another, they're probably all speaking the same language (English, for our benefit).
I'm not convinced Worf knows a word of English. Belorussian, perhaps. But not English. And certainly Kira would take pride in not knowing the language of her new oppressors.
The corollary to that is that languages ought to be dead as a thing. If you use the UT with your colleagues, why not with your family? Why learn even your native language?
I've always wondered this as well. The UT seems to know when to translate and when not to translate, but I have no idea how it determines when to and when not to. For example, when Odo says, "Bon appetite" to Kira on the holosuite, shouldn't the UT translate that phrase?
It would be pretty natural for the device to filter out cusswords, which is exactly what we also hear happen. But knowing what word is objectionable already requires the device to be rather well informed. The identifying of Phrases with capital P, such as "Bon appetit", would automatically follow.
Even easier are those cases where a single word or phrase is suddenly spoken in a different language. Surely the machine must realize this is for a reason? Thus, when Spock suddenly brings up a Vulcan expression such as pon farr, no attempt at translation is made. And conversely, when Kor, Koloth, Kang and Dax have banter between themselves in obvious Klingon and this gets translated to English for our benefit, a single sudden English word such as "Iceman" logically gets translated to Klingon instead.
Computers of the future aren't stupid. Heck, even the doors probably hold doctorates. Leaving the AI
out of your shuttlecraft keys just isn't worth the hassle. Having it in your UT ought to be a yes-brainer.
Timo Saloniemi