It’s available for download!!!
World Cup flag background image complication provider
https://play.google.com/store/apps/details?id=dynalogix.worldcupflags
► It’s paid, but starting 0am GMT (in about 25 minutes) it will become free for an introductory period
► there is no phone app, install it from the Play Store on the watch (or using the web version of Google Play)
► to help you find it in the Play Store on the watch go to “Add complication” in Bubble Clouds and scroll down to “World Cup backgrounds”
► it works best with the newest version 9.05 of Bubble Clouds
► but it should also work well with any other watch face that allows you to set the background color using external complication providers
► as always, please let me know if you find any problems!

It has a launcher icon:
► …which recommends to get Bubble Cloud if not installed 😉
► if you haven’t set a background yet, it takes you directly to the background complication part of Bubble Cloud
► after setting a background, use the launcher icon to fine tune the background’s brightness and contrast
► this also works with other watch faces, in fact the app save your flag selection and preferred settings for each watch face separately
► Trick: some watch faces don’t update the background image after you adjust the brightness/contrast: you can force them to update by switching to a different watch face and back. (this is not necessary with Bubble Clouds of course)

Please leave a nice review in the Play Store if you like the idea.