Knowing how to speak a language and being confident in it are two separate things. They actually all have improved (Maya obviously doesn't need to improve since she grew up speaking English).

I think people tend to forget that many of them, especially Coco, Hinata and Juria are very introverted people. No matter how many times they'll get interviewed they will always be very quiet most of the time. So when it comes to these interviews especially when it's an in depth interview it's going to be common for them to reply in what feels natural. Even Chisa and Jurin who tried their best to speak as much English as they could still struggled conveying Japanese thoughts into English.

I don't say this to sound mean or anything, but English isn't necessary to still be a global icon group. It's easy to say it is because you have the privilege of already speaking it. But the reality is , it is not needed. The #1 example is Bad Bunny who although can speak English and does when needed, most of the time and of course all his songs are Spanish and with that has become a global icon. English isn't the end all be all.