그들은 "노력하면". 영어를 잘 하게될 것이라 믿었고, (그들자신뿐만아니라)
"어느누구도(누구나다)" 노력하면 영어를 잘 하게될 것이라고 믿었다.
-> They believed that, with their own efforts, they could ultimately speak English well and that anyone could do that as well.
그리고, "긍정적인 생각"이라는 것을 언급해주고싶으면 다음과같이 써도 괜찮은가요? ^-^ -> They revealed that optimistic beliefs that they could ultimately speak English well and that any one could do that as well.