A Translation circuit is a system on the TARDIS that can apparently translate almost every language in the known Universe, both spoken and written. Another feature of the circuit is that it can work the opposite.
When the Doctor and Donna arrived in Pompeii, Donna wondered what would happen if she spoke Latin. It turns out that if she speaks Latin to a Roman whilst the translation circuit was on, they would mistake you for being Celtic, or Welsh. (DW: The Fires of Pompeii)
Among the languages that the circuit cannot translate is that of the judoon, (who are shown to speak in a distinct pattern of single-syllable words ending in an 'O' sound) but they have been shown to be capable of speaking English. (DW: Smith and Jones and The Stolen Earth) The circuit cannot decipher the words of the Beast. (DW: The Impossible Planet) In some cases the circuit may cease to function, such as when the Doctor is in a post-regenerative coma. (DW: The Christmas Invasion)
When the Doctor traveled to 18th Century France, the translation circuit was able to allow the Doctor to communicate with people who are presumably speaking French. Even when separated from the TARDIS, the Doctor's speech continues to be translated. (DW: The Girl in the Fireplace) In present day Earth, the circuit does not translate between terrestrial languages. Likewise daleks who invade Germany appear to speak in German. (DW: Journey's End)
Once, all visible writing in the planet of Shan Shen was changed to the words "Bad Wolf", including the writing on the TARDIS itself. Seemingly it was only for the Doctor and Donna, as passers-by were not alarmed. This was probably the work of Translation circuit, as a warning sign. (DW: Turn Left)
