Update
The original answer is no longer correct. The developer’s guide now states:
“If you would like to use “localhost” for development, you must add it to the list of domains.”
This will only work if you access localhost using 127.0.0.1/... rather than localhost/....
The original answer is preserved below.
According to the reCAPTCHA Developer’s Guide:
“localhost domains are no longer supported by default. If you wish to continue supporting them for development you can add them to the list of supported domains for your site key. Go to the admin console to update your list of supported domains. We advise to use a separate key for development and production and to not allow localhost on your production site key.”
In other words, simply use the same key.