Telepathic circuits

Telepathic circuits are a component of TARDISes that allow communication with the Time Lords. (DW: Frontier in Space, The War Games). Additionally, this is the element of the TARDIS systems that allows its occupants to understand and speak languages other than their own. The TARDIS's Time Lord seems to be an integral part of the telepathic circuits; if the Time Lord is unconscious, the translation effect may not work correctly. Apparently, the TARDIS translates certain cultural expressions of the speaker into the contemporary and corresponding phrases of the listener's language and native time period, such as in the case of "Sycorax rock!". (DW:The Christmas Invasion)

While the Doctor's companions hear and speak their native languages and do not perceive the translation effect, the Doctor himself is aware of the translation and whatever local language is being spoken. (DW:The Fires of Pompeii, The Girl in the Fireplace) Should someone benefitting from the translation effect attempt to override it and deliberately speak the local language, it comes across to the locals as garbled. It also seems that the listener (at least in Earth's past, present, and future) hears something similar to the language of the speaker's native culture in that time period. It is also possible that the listener simply hears the native language of the speaker without comprehending it. (DW:The Fires of Pompeii)

The translation circuits translate writing as well as speech. (DW:The Fires of Pompeii) They do not work on exceptionally old and/or forgotten languages. (DW:Four to Doomsday, The Impossible Planet)