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The MaxMind GeoIP database contains some extra codes that we should define in core/DataFiles/Countries.php and lang/en.php.
At the same time, we should re-align the country-to-continent mapping using www.maxmind.com/app/country_continent. This shifts the responsibility for "accuracy" to a third-party.
Another issue that arises is that the GeoIP query appears to sometimes return an empty string (instead of 'xx' or 'unk'), resulting in UserCountry_country_ and UserCountry_continent_.
(In ) fixes #1726 - add Antarctica and Central America; use MaxMind's country-to-continent mapping (with the exception of Central America which it groups with North America).