To improve Visitor Country detection, you should configure Geo Location in Piwik (click to learn more).

If you are not using Geo Location, Piwik is guessing your location from the language locale disclosed by the browser, e.g. en-us -> country is set to USA, fr-fr -> country is set to France, etc. In some cases, Piwik will guess the location when the region is omitted, e.g., fr -> France. This can be disabled from config/global.ini.php (or overridden in your config.ini.php):

[Tracker]
enable_language_to_country_guess = 0

If you want to improve the country detection accuracy in Piwik, setup Geo Location now.