Translation circuit
From Tardis Wiki, the free Doctor Who reference
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.
At the end of Turn Left episode, all writing on and around TARDIS is changed into the words 'Bad Wolf', seemingly only for the Doctor and Donna. It can be assumed this is the work of Translation circuit.
