I suspect that
Charlize found her original ln pronunciation too unfamiliar to most Americans and chose to make life easier for them, and therefore for herself!
Willemien is the male name
Willem (=
William) feminised. And, Afrikaans being close to
Dutch and
German, the W sounds like a V and the i like an uh, or a schwa. And the -mien is like the
Min in
Minnie, the well-known Mouse. There's a light stress on vil and the main stress on MIN.