She is the girl who he wants to talk to. (who은 목적격 인칭 관계대명사 whom 대신 쓰인것이고.)

This is the girl I spoke of yesterday (who대신 that 사용 가능합니다. 이때 that은 또한 생략가능합니다.)

모두 틀린 문장이 아닙니다.

* that이 who을 대신함.
I like people that smile a lot.
Do you remember the people that we met in Italy?

* 목적격 관계대명사 that 생략 가능
- Do you remember the people we met in Italy?

- I forget most of the films I see.
- All I want is your happiness.