I don't know if it works with google maps, but there is a web service that returns a country code and takes as parameters the lat and long.

Here is an example: http://api.geonames.org/countryCodeJSON?lat=49.03&lng=10.2&username=demo

Returns a JSON data: {"languages":"de","distance":"0","countryCode":"DE","countryName":"Germany"}

I also found a little description:

The iso country code of any given point.

• Webservice Type: REST

• Url: ws.geonames.org/countryCode?

• Parameters: lat, ...$]*]