Unfortunately the whole Game Centre pages in App Store Connect are pretty terrible :-/. Well due for some attention Apple.
I'm on a 2020 M1 MacBook Air using Safari in full screen mode and I'm unable to view the popup when I try to add another language.
One tip I can provide that does work is to toggle the maximise window button on the browser (e.g. Safari). I find this is often enough to redraw the screen and bring the popup to actually being displayed on-screen.
Good luck!