How Does the Universal Translator Decide What to Translate?

[XZ923]

That raises another issue with the UT, since we're questioning how it works. It's quite common for English speakers to use foreign phrases for effect. But the sentence would loose something if the UT translated the phrase. So how does it know to NOT translate "a priori", "voilà", "quid pro quo", "je ne sais quoi" or "c'est la vie"?

Similarly, I sometimes watch foreign films with English subtitles and I'm amused to hear vulgarity in English. Apparently that's common practice. Since they're swearing in English it's not clear what the UT should do with it.

Perhaps the UT is programmed to ignore certain phrases? Or maybe it's simply programmed to not translate mixed-language sentences. Or the programming heuristics are "smart" enough to know when to allow a phrase like this to go by un-translated. We are talking about 24th century technology, after all. Compare computer programming technology today to computer programming in 1917, then multiply by 3.