I've just done a search about Vivaldi and Wayland. https://wiki.archlinux.org/title/Vivaldi mentions that X11 is the default one but that it can be override by selecting auto in chrome://flags, Preferred Ozone platform. I tried this in Wayland and Vivaldi now uses an azerty keyboard. So under wayland, it is correctly detected and the locales are correctly taken into account by vivaldi. Another proof that the bug is on Xwayland's side ?