Translation circuit

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)

So far, the only known langauge that the circuits can't translate are the words of the Beast. (DW: The Impossible Planet)

Once, all writing on and around TARDIS is changed into the words 'Bad Wolf', seemingly only for the Doctor and Donna. This could be the work of Translation circuit. (DW: Turn Left)