It was in The End of the World:

Rose: They all speak English.
The Doctor: No - you just hear English. It's a gift of the TARDIS. Telepathic field, gets inside your brain - translates.
Rose: It's inside my brain?
The Doctor: Well, in a good way.
Rose: Your machine gets inside my head. It gets inside and it changes my mind, and you didn't even ask?
The Doctor: I didn't think about it like that.