Stefano’s approach is correct. In Anvil, you would have two repeating panels:
self.repeating_panel_countries
self.repeating_panel_locations
Whenever a user adds/removes a country via the first repeating panel, you would use the uplink to find locations. In pesudo-code:
SELECT * FROM LOCATIONS WHERE country IN (“country1”, “country2”)
Then have Anvil set the items on the second repeating panel
self.repeating_panel_locations.items = {locations}