This took me ages on a Friday evening (yes, I know I live a crazy live) so I thought I'd make a note on what happened so I can do it again in highly unlikely event of technology disappointing me again. I know, right? No precedent for that.
This worked for me on a Huawei p10 plus. I suspect it'll work on other Huawei devices. Probably not on other Android devices as I think it plays with Huawei specific software.
When downloading something from the Play Store it gets stuck on "Downloading" forever with that maddening not-progress bar going round in circles, promising treats and excitement but delivering more scrolling bar. Maddening. Forever.
Standard Android fix
Apparently some people have had success juggling stopping the Play Store (Settings -> Apps -> Play Store -> Force Stop) and clearing the cache (above, then Storage -> Clear Things) then rebooting. You can also try removing the Play Store updates (same again then hit the menu icon in the top right and select "Uninstall updates"). Then reboot again.
None of this worked for me though.
Supermagic Huawei p10 plus fix
There is a Huawei feature which kills background process to save battery power. Hurrah! Apparently it also stops the store downloading anything. Less hurrah. Not that it'll be clear, of course. Maddening download progress bar. It must die.
Apparently it works if you just disable the Google and Huawei bits but finding this took hours and frankly I couldn't be bothered to narrow it down. So instead, I killed the whole thing.
Phone Manager -> Lock screen cleanup -> Close all to enable for everything, then Close all again to disable for everything.
Then reboot. Because it's technology. So you turn it off and on.
But then the Play Store will work! You can download apps which will break in other ways!