Thing is, modern speech doesn't use only words that is genuine to the language the conversation is held in. It's quite obvious that phrases like "Sic semper tyrannis" carry a meaning that is not conveyed by a litteral translation and would need some explaining to do in a non-literal 'translation'. Which is why it's nowadays used as a standing phrase, in latin, even though people rarely speak latin anymore. It's something I notice oftentimes with german words in the english language. While 'verboten' translates relatively nicely to 'prohibited' or some such words, the connotation is slightly different because it is borrowed from german. It's a well known effect, as the proverb "Omnia dicta fortiora si dicta Latina." documents.
So, I think it wouldn't make sense that a translator would be able to circumvent magically such situations. (Of course, though, we might all sound in translation by the translator like the Elcor in Mass Effect. - [Sadly:] I wouldn't be exhilarated by that, though.)
Also, putting in the odd word is simply a way of creating a certain flavour. The use of a Napaani greeting makes a lot of things clear with a simple, single word. It's a way to express the way our characters speak in a way that relying solely on proper english words, phrases and sentences wouldn't allow.
Tl;dr: Oftentimes it doesn't merely matter what our chars say, but also how they say it.