How to create a multi-language website in GatsbyJS?

# Npm users
npm install --save react-intl

# Yarn users
yarn add react react-intl

const languages = ['en', 'pl', 'es'];
const defaultLanguage = 'en';

exports.defaultLanguage = defaultLanguage;
exports.languages = languages;

import {defaultLanguage, languages} from "./i18n";

const getTranslatedPath = (pathname, to) => {
  const currentPageLanguage = getCurrentPageLanguage(pathname);

  let languagePath = '';
  const isDefaultLanguage = defaultLanguage === currentPageLanguage;
  if (!isDefaultLanguage) {
    languagePath = '/' + currentPageLanguage
  }


  let outputPath = `${languagePath}${to}`;

  const hasTrailingSlash = outputPath.endsWith("/");
  if (!hasTrailingSlash) {
    outputPath += "/";
  }

  return outputPath;
}

const getCurrentPageLanguage = (pathname) => {
  const pathElements = pathname.split("/");
  for (let element of pathElements) {
    for (let language of languages) {
      if (element === language) {
        return language;
      }
    }
  }
  return defaultLanguage;
};

export {getTranslatedPath, getCurrentPageLanguage};

import React from "react";
import {Location} from '@reach/router';
import {getCurrentPageLanguage, getTranslatedPath} from "./linkUtils";

const LinkTranslated = ({children = [], className = "", href = "/"}) => {
  return (
    <Location>
      {locationProps => {
        const {pathname = ""} = locationProps.location;
        return <a className={className}
                  href={getTranslatedPath(pathname, href)}
                  hrefLang={getCurrentPageLanguage(pathname)}
        >
          {children}
        </a>;
      }}
    </Location>
  );
};

export default LinkTranslated;

import React from "react";
import {IntlProvider} from "react-intl";

function SimpleLocalize(props) {
  const language = props.pageContext.language;
  const messages = props.pageContext.messages;
  return (
    <IntlProvider
      locale={language}
      messages={messages}>
      {props.children}
    </IntlProvider>
  );
}

export default SimpleLocalize;

{
  "en" : {
    "welcome-on-our-website" : "Welcome on our multi-language website",
    "hello-world" : "Hello World!",
    "about-us" : "About us",
    "learn-more-about-us" : "Learn more about us",
    "homepage" : "Homepage"
  },
  "pl" : {
    "welcome-on-our-website" : "Witamy na naszej wielojęzycznej stronie internetowej",
    "hello-world" : "Witaj Świecie!",
    "about-us" : "O nas",
    "learn-more-about-us" : "Dowiedz się więcej o nas",
    "homepage" : "Strona główna"
  },
  "es" : {
    "welcome-on-our-website" : "Bienvenido a nuestro sitio web multilingüe",
    "hello-world" : "Hola, mundo.",
    "about-us" : "Sobre nosotros",
    "learn-more-about-us" : "Más información sobre nosotros",
    "homepage" : "Página web"
  }
}

const messages = require("./i18n-translations.json")
const {languages, defaultLanguage} = require("./src/i18n");

exports.onCreatePage = async ({page, actions}) => {
  const {createPage, deletePage} = actions;
  return new Promise((resolve) => {
    let path = page.path;
    deletePage(page);

    for (let language of languages) {
      const isDefaultLanguage = language === defaultLanguage;
      if (!isDefaultLanguage) {
        path = '/' + language + page.path;
      }

      const pageForLanguage = Object.assign({}, page, {
        originalPath: page.path,
        path: path,
        context: {
          language,
          messages: messages[language]
        }
      });
      createPage(pageForLanguage)
    }
    resolve()
  })
};

import React from "react";
import {FormattedMessage} from "react-intl";
import LinkTranslated from "../LinkTranslated";
import SimpleLocalize from "../SimpleLocalize";

function IndexPage(props) {
  return (
    <SimpleLocalize {...props}>
      <h1>
        <FormattedMessage
          id="hello-world"
          defaultMessage="Hello World!"
        />
      </h1>

      <p>
        <FormattedMessage
          id="welcome-on-our-website"
          defaultMessage="Welcome on our multi-language website"
        />
      </p>

      <LinkTranslated
        className="btn btn-link"
        href="/about"
      >
        <FormattedMessage
          id="learn-more-about-us"
          defaultMessage="Learn more about us"
        />
      </LinkTranslated>
      <ul>
        <li><a href="/">English (Default)</a></li>
        <li><a href="/pl/">Polish</a></li>
        <li><a href="/es/">Spanish</a></li>
      </ul>

    </SimpleLocalize>
  );
}
export default IndexPage

import React from "react";
import SimpleLocalize from "../SimpleLocalize";
import {FormattedMessage} from "react-intl";
import LinkTranslated from "../LinkTranslated";

function AboutPage(props) {
  return (
    <SimpleLocalize {...props}>
      <h1>
        <FormattedMessage
          id="about-us"
          defaultMessage="About us"
        />
      </h1>

      <LinkTranslated
        className="btn btn-link"
        href="/"
      >
        <FormattedMessage
          id="homepage"
          defaultMessage="Homepage"
        />
      </LinkTranslated>
    </SimpleLocalize>
  );
}
export default AboutPage;

## npm
npm install --save-dev @formatjs/cli

## yarn
yarn add -D @formatjs/cli