Sorry this is a tome. I think this I'm finally winding down on the subject.
^ I think that would be fine if the game were to be set entirely in Tevinter and had only Tevene-speaking persons in it.
Let's take a movie like, say, The Hunt for Red October**, which has one of the better translation transitions in it that I've seen in film. Near the beginning, the Russian scenes in the movie have English subtitles, and there is a scene where a Russian guy is reading a passage from a book in Russian. The camera slowly zooms in on his mouth as he's reading. In a single breath, from one word to the next, he switches from Russian to English, the subtitles disappear, and the viewer understands (or, is supposed to understand) that they are still speaking in Russian, but we are hearing English for our own benefit. The Russian-only scenes stay in this way until they meet the Americans, during which they speak Russian again and we again have the subtitles.
And to again bring in a Star trek reference, in Star trek VI: The Undiscovered Country is a trial scene in a Klingon court. The spoken language switches from Klingon to English at one point, but Capt. Kirk and co. are still holding the translator box to their ear because the language is still supposed to be Klingon. There is actually a bit of a bizarre moment during the trial where the Klingon guy questioning Kirk yells, "Don't wait for the translation, answer me now!" and the viewer (well, I do at any rate) is left to wonder, "Er... if he can't understand you [because you're speaking in Klingon], how is he supposed to know that you just told him not to wait for the translation, without waiting for the translation?" But if he actually yelled that line in English, how is the viewer supposed to know that if he was supposedly speaking in Klingon in the previous moment? o_O
You can think of it in another way: if everyone is speaking Tevene, but we are hearing English, why would there then be sprinklings of Tevene? If there were, that would just lead one to assume that everyone was speaking English (or Trade, as the case may be) and these certain individuals switched to Tevene for whatever purpose. I understand you, you understand me, we understand each other and we're all speaking the same language; so there wouldn't then suddenly be a different sounding language that is actually the language we're supposed to be speaking.
For a final example, I'll point to Doctor Who where the above was used to humorous effect. The T.A.R.D.I.S. -- the thing/ship they travel in -- has a translation circuit that affects the Doctor and companions and enables them to understand the language of wherever they travel to, and likewise the natives can understand them. In the episode The Fires of Pompeii (s4,e2), the Doctor and companion travel to Ancient Rome. After arriving, his companion learns about the translation effect and wonders what would happen if one were to speak Latin to one of the Romans. She does so by saying the phrase veni, vidi, vici, and the Roman then yells that he can't understand Gaelic (or something along those lines). This joke is repeated a couple of times during the episode.
We can use the Dalish DAO origin as a comparison. Those Dalish elves are speaking in Trade with the various Elvish words and phrases thrown in here and there as we're used to. Now, what I CAN see would be something similar for a Tevinter setting with the Elvish substituted for Tevene, which would of course necessitate some more development beyond some swear words and an endearment. I think it would work particularly well if we get a glimpse into altus society.
BUT I can tell you from reading works like Jane Eyre where there is liberal use of French: as a non-French speaker/reader it can get frustrating because it feels like I'm missing something. Nothing really important or plot related, but that ignorance eventually grates as the examples continue to appear.
** I actually read your TV Tropes link after writing all this out, and found to my amusement the exact same example from The Hunt for Red October. The TV Tropes is incorrect about the meeting of the Americans and Soviets in that there are again (some, not full translation of all dialogue) English subtitles, such as, "He's turning green," after a cigarette is offered and taken.
All of that, and my previous postings, aside, I do agree that it is unfortunate that Tevene is less common than originally believed to be. 