Translation circuit

A translation circuit was a part of the Doctor's TARDIS that allowed for the instantaneous translation of most languages spoken or written in the universe.

Basic functionality
As the Ninth Doctor once told Rose Tyler, the translation circuit was a "gift of the TARDIS, a telepathic field that gets inside your brain — translates." (DW: The End of the World) The Doctor himself was a part of the circuit, without whom the circuit was broken. When the Tenth Doctor was incapacitated due to post-regenerative crisis, for example, Rose lost access to the circuit, and could no longer understand languages she did not personally know. (DW: The Christmas Invasion)

Exactly who could take advantage of the circuit, and under what circumstances, was variable, suggesting the Doctor could change its settings in some way. The Fourth Doctor told Sarah Jane in Renaissance Italy that it was a "gift of the Time Lord" that he allowed her to share. (DW: The Masque of Mandragora) During the incident with the Sycorax on Christmas 2006, the translation circuit worked for all humans on the Sycorax ship, even though he was not, at the time, apparently aware of their presence. (DW: The Christmas Invasion) On yet another occasion, the Seventh Doctor told Elizabeth Klein that the telepathic field was limited to a certain radius around the TARDIS. (BFA: Survival of the Fittest)

The Doctor once mentioned that the translation systems had a swear filter that prevented the passengers from hearing any swear words; even an angry cavewoman's rants were translated as her saying 'Blinking' rather than a more apt phrase. (NSA: Only Human)

The translation circuit was apparently capable of translating for creatures without a language. The Doctor was able to barely communicate with a Krafayis, which were only of animal-level intelligence. (DW: Vincent and the Doctor)

Multilingual individuals were sometimes capable of identifying what language was being spoken. When the Fourth Doctor spent time conversing with Emmeline Neuberger, a native German woman who also spoke reasonably good English, he responded to her with equal ease when she spoke to him in English and German without showing any sign that he noticed her change in language, Emmeline noting that she couldn't tell what language he was addressing her in. (PDA: Wolfsbane) The Doctor tended to have more luck in identification. The Doctor identified the mercenary trader Garron as originating from Somerset simply by hearing his accent, despite the fact that he was on the planet Ribos at a time before it had become aware of other worlds and hence would have no reason for an Earth native to visit it. (DW: The Ribos Operation) On another occasion, The Doctor used Madame de Pompadour's particular French accent to identify the century in which she lived, while his companions were unable to tell that she was even speaking French. (DW: The Girl in the Fireplace)

The TARDIS translations were closer to the original language than other translators. Selachian battlesuits had built in translators, giving them a harsh voice. When removed from their armour and in range of the TARDIS, their speech sounded more melodic, closer to the Ockoran's natural song-like language. (PDA: The Final Sanction)

The TARDIS translator gave some individuals noticeable accents. It made Elizabeth Klein, a German-speaker, perceive the Doctor as having a "stuffy Prussian accent". (BFA: Survival of the Fittest) Vincent van Gogh, a native Dutch speaker, assumed that Amy was from Holland due to her having a similar accent to his own (both sounding Scottish in English, and therefore presumably both sounding similar in French). (DW: Vincent and the Doctor)

It has been suggested on some occasions that the TARDIS translation also slightly modifies peoples' minds so that they do not consciously register that they should be unable to understand the languages they are listening to. The Doctor was able to deduce that Sarah had been hypnotised when she directly asked how she was able to understand Italian during a trip to the fifteenth century. (DW: The Masque of Mandragora) On another occasion, when the Brigadier asked how he was able to understand German during a trip to a party thrown by the Nazis in Hitler's honour in 1942, the Sixth Doctor reflected that the champagne the Brigadier had drunk was probably responsible for him asking the question. (PDA: The Shadow in the Glass)

Untranslated languages
Although the circuit could translate a vast array of languages in the universe, there were some instances where the circuit failed. Languages which were too old (DW: The Impossible Planet) or too complex would not be translated (NSA: Night of the Humans). In other instances, if the idea was too complex translation would be incomplete or faulty, an example being when the Doctor used the phrase "We will have been here before" to describe the TARDIS arriving at a crater on the Moon in the early twenty-first century when the TARDIS would visit that location in 1878 in its personal future (PDA: Imperial Moon)


 * Gallifreyan was not translated by the TARDIS. (DW: The Time of Angels, A Good Man Goes to War)
 * Presumably, the Time Lords felt no need to translate their own language.


 * The circuit also failed to translate both German and French while on a planet controlled by the War Lords, while it did translate the War Lords themselves. (DW: The War Games)
 * The Foamasi language went untranslated. (DW: The Leisure Hive) This was apparently due to the complexity, as the language stimulated the visual cortex of the brain, meaning Foamasi effectively saw their language. (NA: Sleepy)
 * The TARDIS was unable to translate for Kurkutji, though Tegan Jovanka was able to speak the language normally. (DW: Four to Doomsday)
 * Golosian was too complex to be translated. (BFA: Bang-Bang-A-Boom!)
 * The language used by the Disciples of the Light was too old to translate, being from previous universe. (DW: The Impossible Planet)
 * The language of the Judoon wasn't translated by the TARDIS. (DW: Smith and Jones, The Stolen Earth)
 * Two out-of-universe explanations were given. In the original script for The Stolen Earth, shown in Doctor Who: The Writer's Tale, the Judoon were "too thick". Steven Moffat claimed it wasn't a language but code, being a "military verbal shorthand".


 * The Hervoken language couldn't be fully translated, though the essence was understood. (NSA: Forever Autumn)
 * Whilst visiting Vincent van Gogh, the Doctor's TARDIS was covered in posters which remained in French. (DW: Vincent and the Doctor)
 * The Sittuun language was too complex to be translated. (NSA: Night of the Humans)
 * A minotaur-like species couldn't be translated properly. Its words sounded like roars and grunts and the Eleventh Doctor had difficulty understanding several words. (DW: The God Complex)

Companions
When the Doctor and Donna arrived in Pompeii, Donna wondered what would happen if she spoke Latin to the locals, whose native language was Latin. Instead of hearing Latin, at least one native speaker of Latin believed her to be Celtic. (DW: The Fires of Pompeii)


 * What exactly happened is unclear. The TARDIS may have translated the Latin back into English, which the speakers thought was Celtic. The words may have not been translated at all, but Donna's pronunciation may have sounded foreign.

Most of the Doctor's companions were native speakers of English, and the Doctor himself was a "master of English" — indeed a fan of the language. (BFA: ...ish) However, he did occasionally travel with non-English-speakers. Elizabeth Klein, a native German speaker, required the circuit just to communicate with the Doctor. (BFA: Survival of the Fittest)

The circuit could apparently be influenced by the passengers who travelled in it. When the Doctor, Fitz and Anji landed on a world where they encountered the 'mooncalfs'- people born with some kind of genetic deformity, regarded as an abomination in this world-, Fitz recalled the term from his childhood as referring to someone who was either a bit slow or someone who was a freak, speculating that the TARDIS translator chose a word that he knew to describe them because it was more 'tuned in' to his 'wavelength' given that he had spent longer travelling with the Doctor than Anji. (EDA: Vanishing Point)

Anji had a particularly unusual relationship with the translation circuit. She actually learned a new language whilst travelling with the Doctor. Caught in the midst of the Spanish Civil War, she used her pre-existing fluency in French to quickly assimilate Catalan while she was in Barcelona. (EDA: History 101) It was unknown how she was able to experience traditional language learning whilst still nominally linked to the TARDIS. Moreover, it was unclear how the translated circuit worked in her mind, since she was naturally bilingual from the start of her association with the Doctor.

Before travelling with the Eighth Doctor, Mary Shelley had been trying to learn German. When she travelled to Austria with the Doctor, she assumed that she was just picking up German faster when she could easily read the newspaper. (BFA: The Silver Turk)

The Doctor
It is unknown how much the Doctor himself used the translator. The Doctor once claimed that he spoke 9 billion languages, but this may be only with the TARDIS. (DW: The Parting of the Ways) At the very least, he was able to converse with Tritovores on San Helios, even though the TARDIS was on Earth. (DW: Planet of the Dead) On the other hand, it is known that the Doctor was unable to understand the language of the Vrill without the TARDIS, which used smell instead of sound. (BFA: Survival of the Fittest)

It was possible that the translation circuits draw upon the Doctor's vast knowledge to translate; this is supported by the fact that the translation circuit ceased working with the Doctor was incapacitated (DW: The Christmas Invasion), or when the Doctor didn't know the language on the wall (DW: The Impossible Planet).

Furthermore, there were times when he was able to interject non-English words into his otherwise English speech — such as the phrases "cogito ergo sum", "nul points", "molto bene" and "Allons-y!" — without the circuit translating the phrase. The phrases were sometimes untranslated even when at least some of the recipients were not likely native speakers of English, in which the phrases were arguably loaned. (DW: Revenge of the Cybermen, The Parting of the Ways, Midnight, Voyage of the Damned, The End of Time)

Behind the scenes
The TARDIS crew's ability to understand other languages wasn't explained in any story in any medium until The Masque of Mandragora. Even so, that was the only story in the whole of the 1963 version of Doctor Who to even touch upon the subject of language translation. The issue has had much greater prominence in the BBC Wales programme, which has then had an impact on the Big Finish past Doctor stories.