What’s the best way to get device locale in react native (iOS)?

There’s no need for an external library. You can find what you’re looking for in React’s Native Modules

import { NativeModules } from 'react-native'

// iOS:
const locale = NativeModules.SettingsManager.settings.AppleLocale ||
               NativeModules.SettingsManager.settings.AppleLanguages[0] // "fr_FR"

// Android:
const locale = NativeModules.I18nManager.localeIdentifier // "fr_FR"

To test this, I changed the language on my device to French. Here’s a sample of what you’ll find in the NativeModules.SettingsManager.settings object related to locale:

{
    ...
    AppleKeyboards: [
        "fr_FR@hw=US;sw=QWERTY",
        "en_US@sw=QWERTY;hw=Automatic",
        "es_419@sw=QWERTY-Spanish;hw=Automatic",
        "emoji@sw=Emoji"
    ]
    AppleLanguages: ["fr-US", "en-US", "es-US", "en"]
    AppleLanguagesDidMigrate: "9.2"
    AppleLocale: "fr_FR"
    NSLanguages: ["fr-US", "en-US", "es-US", "en"]
    ...
}