To me, my internal world is something that I could not describe in words. At best, it can be approached obliquely and is only hinted at in anything that I say/do. A metaphor for that would be spirituality. I would describe it as a wholeness that is neither coherent nor consistent, but is undeniably complete.
The external world is something that can be broken down into an infinite number of components and be made coherent by my inner world choosing which bits of information to assimilate.
While my inner world affects what I view of the outer world and my interpretation of it, the external world and experiences also shape the inner world of today and affect how I view the external world of tomorrow. As such, while I experience my inner world and external world as being separate from each other, there is undeniably an interplay between them.
These are ideas that I came up with while thinking about the nature of identity and self-definition.
I haven't read other responses yet, but with Jungian Functions, the internal world would be the subjective reality or the wider reality you infer. The external world would be the objective or actual reality in front of you. I think Jung said Ne is sort of an anomaly in this sense.
Freude, schöner Götterfunken Tochter aus Elysium, Wir betreten feuertrunken, Himmlische, dein Heiligtum! Deine Zauber binden wieder Was die Mode streng geteilt; Alle Menschen werden Brüder, Wo dein sanfter Flügel weilt.
I don't think it's as simple as the mind being the internal world and the external world being 1-the mind. The mind counts as part of the external world in a lot of situations (possibly at the same time it's part of the internal world).