Something similar to [NSLocale currentLocale] in Objective-C.
import { Platform, NativeModules } from 'react-native'
const deviceLanguage =
Platform.OS === 'ios'
? NativeModules.SettingsManager.settings.AppleLocale ||
NativeModules.SettingsManager.settings.AppleLanguages[0] //iOS 13
: NativeModules.I18nManager.localeIdentifier;
console.log(deviceLanguage); //en_US