You need to configure Piwik to use a different method of identifying the location of the visitors to your site. Click on the Settings link at the top of the page after having logged into Piwik with your Admin credentials. Then click on the Geolocation menu option in the list down the left of the page.
The default Geolocation option is to use the language setting of the browser. If the browser is set to English (US), Piwik assumes the visitor is located in the USA. If it’s set to English (UK), Piwik assumes United Kingdom. If it’s German (Swiss), Piwik assumes Switzerland.
To activate one of the other options, you first need to install one of the GeoIP databases from MaxMind. I have set my site to use the GeoIP (PHP) method because that was the simplest to install, and because my site doesn’t see much traffic. Click on the How do I get the GeoIP databases? link above the GeoIP (PHP) option for information on how to download and install the proper database.
MaxMind updates its freely downloadable database on the first Tuesday of the month. Every month, on the first Wednesday, I download the GeoIP (City) database from the MaxMind site. After unpacking the database, the resultant file is renamed to “GeoIPCity.dat” and is then copied to the “misc” directory in the server’s Piwik installation to replace what’s already there.