In Star Trek, there are universal translators on the ship to make communications between the different species possible.
But there are countless times when Klingons talk in Klingon and English, when Picard quotes French or Latin, when characters say a few lines here or there in some foreign tongue to show they know the language.
Why doesn't the translator translate these phrases, too, if all other languages are instantly translated?
From http://en.memory-alpha.org/wiki/Universal_translator (Which got it from TOS 2x02: Metamorphosis)
Responding to Zefram Cochrane's question about the theory of operation, Kirk explained that there are certain universal ideas and concepts common to all intelligent life, and that the translator compared the frequencies of brainwave patterns, selected those ideas it recognized, and provided the necessary grammar. Kirk further explained that the device spoke with a voice, or the approximation of one, that corresponded to the identity concepts it recognized.
Since it's already scanning brainwaves, presumably it can also detect intent (whether you want to be understood or not), and decide whether or not to translate as appropriate.
I don't buy the brainwave explanation. Aside from it being nearly impossible, it doesn't fit in with the rest of the show.
It would have to scan the brain of a an unknown race, understand how it works, assume it uses "brainwaves", know which of those waves controls thought and speech, and figure out what the waves mean. And it's not just translating broad thoughts, it's getting every nuance and word. The translation is often very eloquent. It's just absurd.
And most importantly, it requires mind-reading at a very deep level. Nothing else in any of the series suggests the Federation can read minds. They would have used that for many other things (e.g. is a suspect lying?, is this guy on the viewscreen bluffing or is he going to fire, is my crewmate a clone taken over by an alien). They wouldn't need Troi to guess using fuzzy feelings (they would also be able to hear telepaths telepathing).
The universal translator does not read brainwaves.
While the brainwave explanation was mentioned in one TOS episode, everything else in every other series has clearly shown it's a language learning process. ENT covered this a lot and showed the fallibility of earlier technology.
There are two parts to this. First, is the theoretical nature of the translator and what it would do if you were using one. And second, what we, as viewers, hear and see (subtitles) for storytelling convenience.
When a race is by itself, with no aliens present, a translator could determine that and not translate. As viewers we may see subtitles or we may hear English. But we can assume no translation is happening.
When multiple races are present, each speaking their own language, the translator has to hear what you're saying, translate it into every language of those present, and alter the sound wave in the direction of each person listening. We saw this on VOY when they found Amelia Earhart.
Sound research today can already completely block out sounds with alternate waves and project sound in very precise directions. So it isn't hard to imagine the mechanics of this working in a few hundred years.
A translator can accurately determine those present and what language they speak based on their race. We've seen the translator mess this up before when a smaller group of people of a particular race speak a different language. So it can determine you're human, and decide to use English, but can't determine if you're a human that doesn't speak English unless it hears you speaking first.
It would be easy enough to determine (or at least predict very accurately) who you were speaking to in a room based on vocal and body cues. If you're Vulcan near another Vulcan and speaking directly to them, it could choose not translate. We see this sometimes when a Klingon captain will turn to one of his crew and shout a command and we, as viewers, hear it in Klingon (sometime with subtitles), even with people from other races around. We can assume the translator didn't translate this.
With all that in mind, it's easy to see how Picard can speak French, or Hoshi can speak Denobulan. The translator knows your race and what language you speak. When it hears you speaking a different language, it would know that was deliberate and wouldn't translate it.
Also, it seems at least some translators are embedded in your body (Little Green Men, DS9). A translator could learn your preferences and proficiencies over time (and you could set them as well). If you know a lot of Klingon, for example, it could stop translating things you hear all the time, like greetings.
Where this doesn't work is when two people are speaking another language when someone from another race is around and one of them says something like, "Speak <guest language> around the guest." There is no reason why the translator wouldn't translate if the guest was part of the conversation, and no expectation that any other aliens would speak or even know <guest language>. But brainwaves don't solve this either.
I can't think of any examples other than Troi to her mom, but they're speaking telepathically. So that doesn't really count (but it does show that the translator isn't reading brainwaves).
Why doesn't the translator translate these phrases, too, if all other languages are instantly translated?
There are certain concepts in other languages that are so alien to us that the translator is not able to make an approximation.
To give an example. How would you explain Brandenburg Concerto 3 to someone who is not able to hear music, and capture the essence of the song, rather then what is happening?
On top of that you have sentence structure, grammar based on customs, which cannot be determined. For example "안녕" and "안녕하세요" both mean the same thing, but if said in the wrong context can cause confusion or anger in the person you are speaking to. This is why it is mentioned that the universal translator works better the longer it can analyse the language.
In closing, Darmok and Jalad at Tanagra.
Some phrases simply aren't directly translatable. For example, the German word "Todschlagargument" has no direct English equivalent, and as a result, when it's needed, we use a German loadword. This issue pops up a lot with Japanese food.
Some phrases simply don't translate. Why would this be different with alien languages?
Language is mathematical, so they will establish a base, and with AI, learn the language from other languages. Also, somebody in human language had to learn it from scratch without translation material, being on the okanet, analysing phrases said etc, could all be done by analysing news, literature, music (yes, there is a link to language formation) from the current time to the history of the planet (they would had it digitized). So, when meeting a new species for a first time:
Note: theoretically, if the Enterprise analysed the history of the culture in Darmok, it could establish what the metaphors mean and then translate them to English that is structured correctly from our point of view. In terms of sound, the combadge has some directly speaker system to cancel out and project the translations.
I always felt that the Star Trek UT worked directly with the brainwaves. It was similar to Farscape’s translator microbes, it make you FEEL that the other person was speaking in your language. This is actually shown in an episode of Discovery, we hear the Klingon speaking in Klingon with subtitles and once the UT is activated we hear them in English (or whatever human languages is the dub), the Klingon even says “I did not expected you to speak Klingon” and Michael says it is the translator. This would also explain why a Japanese heard people speaking in Japanese in “The 37” and how Picard and Data are capable of going undercover as Romulans into Romulus itself. You don’t really hear anything over the other person’s voice, you hear the person’s voice in your language because the trick is that your brain itself changes the meaning inside your head.
Of course, this doesn’t explains the lip-sync, technically that should still be an issue. But everyone who has seen a dubbed movie (and I don’t mean the terrible bad old times dub of Asian martial art films, but the careful quality dubbing you can see in Western media like between Spanish, English and French) your mind ignores the lack of sync. I guess it could be handwave that unless the language is too different and it takes you a lot of time to say one word your mind just overlooks the lack of sync like watching a dubbed film.
