To Gereon Bock.  The statement you want IS in the Truffaut book.  It
comes at the very end (the last paragraph of Chapter 15, before the
added Chapter 16 of the Revised Edition):

Hitchcock: 'A novel may lose a lot of its interest in [translation], and
a play that's beautifully acted out on opening night may become
shapeless during the rest of the run, but a film travels all over the
world.  Assuming that it loses fifteen per cent of its impact when it's
subtitled and [only] ten per cent when it's well dubbed, the image
remains intact, even when the projection is faulty.  It's your work
that's being shown - nothing can alter that - and you're expressing
yourself in the same terms everywhere.'

- Ken Mogg (Ed., 'The MacGuffin').

Online resources for film/TV studies may be found at ScreenSite