To accept requests over localhost and HTTP (HTTP referrers) in Google Maps API, follow the instructions in the javascript console printed out by the Google Maps API (it tells you exactly what to do):
Google Maps API error: RefererNotAllowedMapError https://developers.google.com/maps/documentation/javascript/error-messages#referer-not-allowed-map-error
Your site URL to be authorized: http://localhost:3000/en
The line Your site URL to be authorized indicates which value you have to set up in Accept requests from these HTTP referrers (websites)
In this case, e.g. for a Ruby on Rails App, it is:
http://localhost:3000/en
But you can do better, by using asterisks for wildcards:
*localhost:3000/*
To set the value to localhost:3000/* does not work!