According to Capcom Japan translator Tom Shirawa, the reason why Nash's name was changed to Charlie for the international release is because the people at Capcom U.S.A did not like it. Nash wasn't an English name and he's supposed to be a U.S. soldier, so they felt that players could not connect with the character and suggested the name Charlie instead.

Capcom eventually decided to merge the two names into 'Charlie Nash' for Street Fighter IV.
Contributed by ZpaceJ0ck0