There are 28 regions in the App Store and each country has its own App Store and rankings. If you already launched your app and it’s growing, the chances are users search for your app in their native language first.
According to Distimo, an app analytics company, localization of your keywords leads to 128% download increase in the first week. Some apps like Abe the Dragon experienced more than 700% boost in growth. Here’s a great post on getting started..