Do you think I will be able to speak English someday?
Do you think I will be fluent in English someday?
どちらも「いつか英語が話せるようになると思う?」という言い方です。
1) Do you think I will be able to speak English someday?
2) Do you think I will be fluent in English someday?
be able to speak English 「英語を話すことができる」
be fluent in English 「英語を流暢に話す」
ご参考になれば幸いです!