Sorry for the late reply (somehow I didn't receive AMO notification by email). Reader mode acts like a system page (like about:preferences, or addons.mozilla.org) so with the limitation of webExtension for now won't allow extensions to interact with it.

EDIT: "quick button" still doesn't work in reader mode, but you can use Sidebar + QD's context menu to translate in reader mode. https://imgur.com/KWbu1ZX