Localization.ts

import { observable, action, computed } from 'mobx';
import { Platform, NativeModules } from 'react-native';
import replace from 'lodash/replace';

enum Locales {
  ru = 'ru',
  en = 'en'
}

type TLocale = keyof typeof Locales;
type TLocales = { [key in TLocale]: string };
type TDictionary = { [key: string]: TLocales };

class Localization {
  @observable private _locale: TLocale;
  @observable private dictionary: TDictionary;

  constructor(dictionary: TDictionary, locale?: TLocale | undefined) {
    this._locale = this.processDefaultLocale(locale);
    this.dictionary = dictionary;
  }

  private processDefaultLocale(locale: TLocale | undefined) {
    if (locale) return locale;
    return Platform.select({
      ios: NativeModules.SettingsManager.settings.AppleLocale,
      android: NativeModules.I18nManager.localeIdentifier
    }).substring(0, 2);
  }

  @computed get locale() {
    return this._locale;
  }

  @action switch(locale: TLocale | undefined) {
    this._locale = this.processDefaultLocale(locale);
  }

  translate(text: string, params: Record<string, any> = {}) {
    if (
      this.dictionary &&
      this.dictionary[text] &&
      this.dictionary[text][this._locale]
    ) {
      return Object.keys(params).reduce((acc: string, param) => {
        return replace(acc, new RegExp(`{${param}}`, 'g'), params[param]);
      }, this.dictionary[text][this._locale]);
    }
    return text;
  }
}

export default new Localization({
  "someText": {
    "ru": "Какой-то текст",
    "en": "Some text"
  }
});

@observer
export default class Example extends React.Component {
  componentDidMount() {
    Localization.switch('ru')
  }

  render() {
    return <Text>{Localization.translate('someText')}</Text>
  }
}