accounts-frontend/src/services/i18n/i18n.js

// @flow
import type {IntlShape} from 'react-intl';
import {createIntl, createIntlCache} from 'react-intl';
import captcha from 'services/captcha';
import locales from 'i18n/index.json';

import intlPolyfill from './intlPolyfill';

const SUPPORTED_LANGUAGES = Object.keys(locales);
const DEFAULT_LANGUAGE = 'en';

function getBrowserPreferredLanguages(): string[] {
    return []
        .concat(navigator.languages || [])
        .concat(navigator.language || []);
}

function detectLanguage(
    userLanguages: string[],
    availableLanguages: string[],
    defaultLanguage: string,
): string {
    return userLanguages
        .map((lang) => lang.split('-').shift().toLowerCase())
        .find((lang) => availableLanguages.indexOf(lang) !== -1) || defaultLanguage;
}

const cache = createIntlCache();

let intl: IntlShape;

class I18N {
    detectLanguage(lang: string): string {
        return detectLanguage(
            [lang].concat(getBrowserPreferredLanguages()).filter((item) => !!item),
            SUPPORTED_LANGUAGES,
            DEFAULT_LANGUAGE
        );
    }

    getIntl(): IntlShape {
        if (!intl) {
            intl = createIntl({
                locale: 'en',
                messages: {}
            }, cache);
        }

        return intl;
    }

    async changeLocale(locale: string = DEFAULT_LANGUAGE): Promise<IntlShape> {
        const { messages } = await this.require(locale);

        captcha.setLang(locale);

        intl = createIntl({
            locale,
            messages
        }, cache);

        return intl;
    }

    async ensureIntl() {
        await intlPolyfill('en');
    }

    async require(locale: string): Promise<{
        locale: string;
        messages: { [string]: string };
    }> {
        const [{default: messages}] = await Promise.all([
            import(/* webpackChunkName: "locale-[request]" */`i18n/${locale}.json`),
            intlPolyfill(locale)
        ]);

        return {
            locale,
            messages
        };
    }
}

export default new I18N();