Is it possible to show the english map instead of the localized map with this addon? This would make it possible to e.g. play dragonflight beta with a working map instead of just green placeholders.

Map data is locale independent. Do you have a screenshot?

See also https://eu.forums.blizzard.com/en/wow/t/dragon-isles-map-not-working-green-map/382758/2

Thank you for the screenshot and link. I checked the game files and the map files don't yet exist for non-enUS clients. You will just need to do what the forum post says and change your locale to enUS and/or wait for Blizzard to fix it.