A Defense of "Does it need saying?" An in-depth look at the Doctor's refusal to say "I love you" to Rose Tyler. I loved this episode and how they tied up Rose and Ten, even if it did break my heart. This explanation is great.
The TARDIS translates it into English. They speak the Queen's English so the TARDIS translates it to the Queens English for them. They understand American English, therefore the TARDIS has no reason to translate.