Localization

English is only one of many languages people speak around the world (thanks a lot, Tower of Babel) and it's polite and sometimes even necessary to translate our app to the languages our users speak. Let's look at one way we can do this in React Navigation - it's not the only way but it'll do the trick. Similar to themes, we will use screenProps. You may also want to use React's context API as demonstrated in the themes guide in order to make it easier to access the translate function from a variety of components.

Setting up a localization library

We'll need to use some kind of library to store our translations and provide a function that gives us access to them, along with handling fallbacks when we don't have a particular language defined. Localization and internationalization (i18n) are often used interchangeably, as in the example below where we get the current locale from expo-localization and use the i18n-js library for managing translations, for no particular reason other than it was available - use whatever you like.

import * as Localization from 'expo-localization'; // or whatever library you want
import i18n from 'i18n-js'; // or whatever library you want

const en = {
  foo: 'Foo',
  bar: 'Bar {{someValue}}',
};

const fr = {
  foo: 'Fou',
  bar: 'Bár {{someValue}}',
};

i18n.fallbacks = true;
i18n.translations = { fr, en };

// This will log 'en' for me, as I'm an English speaker
console.log(Localization.locale);

Wiring up your localization library to navigation

Next let's store our locale in the state of our root app component and then thread it through screenProps to make it available throughout React Navigation.

export default class App extends React.Component {
  state = {
    locale: Localization.locale,
  };

  setLocale = locale => {
    this.setState({ locale });
  };

  t = (scope, options) => {
    return i18n.t(scope, { locale: this.state.locale, ...options });
  };

  render() {
    return (
      <AppContainer
        screenProps={{
          t: this.t,
          locale: this.state.locale,
          setLocale: this.setLocale,
        }}
      />
    );
  }
}

Now in our screens we can use these screenProps as follows:

class Screen extends React.Component {
  static navigationOptions = ({ screenProps: { t } }) => ({
    title: t('foo'),
  });

  render() {
    let { t, locale } = this.props.screenProps;

    return (
      <View style={styles.container}>
        <Text style={styles.text}>
          Current locale: {locale}.{' '}
          {locale !== 'en' && locale !== 'fr'
            ? 'Translations will fall back to "en" because none available'
            : null}
        </Text>
        <Text>{t('bar', { someValue: Date.now() })}</Text>
        {locale === 'en' ? (
          <Button
            title="Switch to French"
            onPress={() => this.props.screenProps.setLocale('fr')}
          />
        ) : (
          <Button
            title="Switch to English"
            onPress={() => this.props.screenProps.setLocale('en')}
          />
        )}
      </View>
    );
  }
}

You can run this example in this Snack. Again, you may want to go further than just passing this through screenProps if you want to make it easier to access the t function or the other screenProps from any React component (and not just screen components that are rendered by React Navigation). Refer to themes and the React documentation on context for help with that.