By default Piwik uses the browser language to determine location. To improve accuracy of country detection (and enable Region and City tracking), go to Piwik > Settings > Geo Location. Then, you can select the appropriate solution for your install. These include GEOIP database from Maxmind (for shared hosting) or a server side solutions such as GeoIP Apache or GeoIP PECL modules. Learn more about Visitors geo location in Piwik.