The TARDIS hates translating. Of all her functions, it is easily her least favorite. There is so rarely any challenge to it. She "hears" the meaning in the speaker's mind, and passes it on to the listener. If she had to listen the way simple, non-telepathic, four-dimensional beings did, that would be different. Translating would be an accomplishment. Anyway, the Doctor knows so many languages, that she suspects he's doing the work himself and ignoring her contribution.
She has noticed before (and will notice again) that tourist lectures are usually even duller to translate than most speech. Translating imagery and figures of speech is one of the few somewhat challenging parts of the task. It seems paradoxical that a speech that is so full of local poetry should be dull, until you realize that part of a good guide's task is to make those hidden meanings plain. So while the TARDIS loves most things about the Doctor's periodic visits to Aqua, the near inevitable gondola trip is usually not one of them.
This time is different. The Undine is very good at her job, which means that she doesn't just have a stock spiel. There is personality and individuality to her tour, but that's not the difference. The Doctor has had/will have many other excellent Undines give him tours. That extra bit of personality elevates the lecture, but still the translation is not as interesting as a street corner conversation about the weather. The best bit, the part she is waiting for, will happen shortly when the gondola pulls into the Grand Canal.
She has never completely understood the sharp line of division four-dimensional beings (even the Doctor) draw between their personal past and future. Still, there is something special about the moment they call "now," that point where the possible futures condense to a single actuality. She has felt time converging on a single particular event all day. One that she has always known could happen, and so in a sense has always already experienced. And yet, there is/was/will be something special about the actual moment when the chain of PossiblePlausibleProbableInevitable ends with an Is.
The gondola turns into the Grand Canal, and the Undine known as the Siren begins to sing.
The magical thing is that, while the words have no meaning, the song does. There is joy and melancholy and the triumph of greeting the dawn after the darkest of nights. The TARDIS has never understood how this is possible, and she doesn't want to. She doesn't think the humans understand it completely either, because all conversation within earshot of the singer ceases. The TARDIS shuts down the now unnecessary translation circuits and listens to the song.
Author's note: I've published this previously on the Doctor Who fanfic website "A Teaspoon and an Open Mind," under the pen name MacA.
If you are reading this because of Doctor Who and you know nothing about Aria, look up "Athena Glory Barcarolle" on YouTube. Kawai Eri's singing is sublime, and this story could not have been conceived without it.