Step by step guide (v9)

Lets start with the sample created in the quick start guide and extend it to be of more use.
react-i18next will run in any environment without you having to do changes to your babel or webpack setup.
Initial situation based on quick start
We have installed the needed i18n packages react-i18next and i18next:
npm install react-i18next@legacy i18next --save
Added following files:
translation.json (/public/locales/en/translation.json)
{
  "Welcome to React": "Welcome to React and react-i18next"
}
i18n.js (/src/i18n.js)
import i18n from "i18next";
import { reactI18nextModule } from "react-i18next";
​
import translationEN from '../public/locales/en/translation.json';
​
// the translations
const resources = {
  en: {
    translation: translationEN
  }
};
​
i18n
  .use(reactI18nextModule) // passes i18n down to react-i18next
  .init({
    resources,
    lng: "en",
​
    keySeparator: false, // we do not use keys in form messages.welcome
​
    interpolation: {
      escapeValue: false // react already safes from xss
    }
  });
​
export default i18n;
Make sure to import i18n.js in index.js:
import React, { Component } from "react";
import ReactDOM from "react-dom";
import './i18n';
import App from './App';
​
// append app to dom
ReactDOM.render(
  <App />,
  document.getElementById("root")
);
Have App.js using the t function for translation:
import React from 'react';
​
// the hoc
import { withNamespaces } from 'react-i18next';
​
function App ({ t }) {
  return <h1>{t('Welcome to React')}</h1>
}
​
export default withNamespaces()(App);
1) Adding more languages
a) Add an additional language file
translation.json (/public/locales/de/translation.json)
{
  "Welcome to React": "Willkommen bei react und react-i18next"
}
b) Add the additional translations on init
in i18n.js:
// ...
import translationEN from '../public/locales/en/translation.json';
import translationDE from '../public/locales/de/translation.json';
​
// the translations
const resources = {
  en: {
    translation: translationEN
  },
  de: {
    translation: translationDE
  }
};
​
// ...
c) Auto detect the user language
As the language is set on i18n.init you either could create some custom code setting the needed language or just use one of the provided language detectors coming with i18next.
For browser usage there is the i18next-browser-languageDetector which detects language based on:
cookie
localStorage
navigator
querystring (append ?lng=LANGUAGE to URL)
htmlTag
path
subdomain
npm install i18next-browser-languagedetector --save
Using it before i18n.init is all needed to get this to work:
import i18n from "i18next";
import detector from "i18next-browser-languagedetector";
import { reactI18nextModule } from "react-i18next";
​
import translationEN from '../public/locales/en/translation.json';
import translationDE from '../public/locales/de/translation.json';
​
// the translations
const resources = {
  en: {
    translation: translationEN
  },
  de: {
    translation: translationDE
  }
};
​
i18n
  .use(detector)
  .use(reactI18nextModule) // passes i18n down to react-i18next
  .init({
    resources,
    lng: "en",
    fallbackLng: "en", // use en if detected lng is not available
​
    keySeparator: false, // we do not use keys in form messages.welcome
​
    interpolation: {
      escapeValue: false // react already safes from xss
    }
  });
​
export default i18n;
Now we already are able to set language based on the browsers set language or by appending ?lng=LANGUAGE to the URL.
d) Let the user toggle the language
Call i18n.changeLanguage is all needed to do.
import React from 'react';
import i18n from './i18n';
import { withNamespaces } from 'react-i18next';
​
function App ({ t }) {
  const changeLanguage = (lng) => {
    i18n.changeLanguage(lng);
  }
​
  return (
    <div>
      <button onClick={() => changeLanguage('de')}>de</button>
      <button onClick={() => changeLanguage('en')}>en</button>
      <h1>{t('Welcome to React')}</h1>
    </div>
  )
}
​
export default withNamespaces()(App);
It's essential to have at least your outer page level / container component wrapped with the withNamespaces or NamespacesConsumer as those are bound to the languageChanged event and trigger a needed rerender.
2) Lazy loading translations
We haven't yet started splitting translations into multiple files (which is highly recommended for larger projects) but we already see that adding more languages would result in bundling unneeded translations into the application.
Why not just use dynamic imports to load the language needed?
This for sure works but comes with one drawback. If you have a change in your translations you will need to rebuild your application and deploy that.
This might not be a problem when starting but at some point you will learn localization is a complete different beast than just adding i18n to your code. You will keep translations as separated from your code as you can - so developers and translators can work as independent as possible.
a) Adding lazy loading for translations
This will be simpler than you think. All needed to be done is adding another package called i18next-xhr-backend and using that.
npm install i18next-xhr-backend --save
i18n.js
import i18n from "i18next";
import detector from "i18next-browser-languagedetector";
import backend from "i18next-xhr-backend";
import { reactI18nextModule } from "react-i18next";
​
// translations are already at
// '../public/locales/en/translation.json'
// which is the default for the xhr backend to load from
​
i18n
  .use(detector)
  .use(backend)
  .use(reactI18nextModule) // passes i18n down to react-i18next
  .init({
    lng: "en",
    fallbackLng: "en", // use en if detected lng is not available
​
    keySeparator: false, // we do not use keys in form messages.welcome
​
    interpolation: {
      escapeValue: false // react already safes from xss
    }
  });
​
export default i18n;
i18next implementation is smart enough to only load needed languages and comes with intelligent deduplications so even multiple load requests for files in different code locations result in one request while notifying all needed requester.
b) Loading multiple translation files
Lets assume your project started to grow and you like to split translations into multiple files.
Without configuration i18next will always load one file (namespace) named translation. Learn more about namespaces here.
You can load them on i18n.init or in code like:
i18n.init({
  ns: ['common', 'moduleA', 'moduleB'],
  defaultNS: 'moduleA'
}, (err, t) => {
  i18n.t('myKey'); // key in moduleA namespace (defined default)
  i18n.t('common:myKey'); // key in common namespace
});
​
// load additional namespaces after initialization
i18n.loadNamespaces('anotherNamespace', (err, t) => { /* ... */ });
But that will load all upfront or in some custom code where you would need to handle manually that the translations where loaded. So there is a better way.
Use the withNamespaces hoc
As you might guess the withNamespaces hocs role is to lazy load the needed namespaces.
page2.json Add more translation files: (/public/locales/en/page2.json)
{
  "Welcome on page2": "Welcome on page2"
}
Page2.js (/src/Page2.js)
import React from 'react';
​
// the hoc
import { withNamespaces } from 'react-i18next';
​
function Page2 ({ t }) {
  return <h1>{t('Welcome on page2')}</h1>
}
​
export default withNamespaces('page2')(Page2);
c) Handle rendering while translations are not yet loaded
In the above Page2 you will notice the initial render will trigger the load for page2.json but also render the page without the translations ready. There will be a rerender when the translations where loaded resulting in the page flickering.
Using the prop tReady
tReady will be set to true when translations are loaded so you can eg. suspend rendering and show a Spinner or another placeholder:
import React from 'react';
​
// the hoc
import { withNamespaces } from 'react-i18next';
​
function Page2 ({ t, tReady }) {
  // just to add some more here we load an additional namespace
  // common for "common" used texts and use that namespace
  // be prefixing it in front of the key "common:"
  if (!tReady) return <p>{t('common:loading')}</p>
​
  return <h1>{t('Welcome on page2')}</h1>
}
​
export default withNamespaces(['page2', 'common'])(Page2);
Set the global wait option
You can configure the withNamespaces / NamespacesConsumer to not render the content until needed namespaces are loaded.
i18n.js:
import i18n from "i18next";
import detector from "i18next-browser-languagedetector";
import backend from "i18next-xhr-backend";
import { reactI18nextModule } from "react-i18next";
​
i18n
  .use(detector)
  .use(backend)
  .use(reactI18nextModule) // passes i18n down to react-i18next
  .init({
    lng: "en",
    fallbackLng: "en", // use en if detected lng is not available
​
    keySeparator: false, // we do not use keys in form messages.welcome
​
    interpolation: {
      escapeValue: false // react already safes from xss
    },
​
    // react-i18next options
    react: {
      wait: true
    }
  });
​
export default i18n;
Now Page2 will be blank until the needed translation files were loaded.
You can set the wait option either globally for all instances or individually like:
withNamepaces('page2', { wait: true })(Page2);
3) Sidequest: natural vs. keybased catalog
a) natural keys
Until now we had organised translation and keys in natural language.
<h1>{t('Welcome on page2')}</h1>
Having translation files like:
{
  "Welcome on page2": "Welcome on page2"
}
This was possible by setting keySeparator: false on i18n.init
The upside of this the code can be more readable but the content of the key might get soon rather different from the value it reflects. You could even go a step further by disabling any fallback language an using the key as fallback - but be aware if you got a typo in the key acting as fallback value you will need to change it.
b) keybased catalog
Some i18n frameworks organise having more technical keys allowing those to be even structured into hierarchies. This is the default in i18next too - so removing keySeparator: false on i18n.init would enable having catalogs and t usage like:
{
  "titles": {
    "page2": "Welcome on page2"
  }
}
accessed like:
<h1>{t('titles.page2')}</h1>
If you prefer natural or keybased is a matter of taste...both can be used with react-i18next. Just avoid mixing those styles.
4) HOCs are dead - long lives the render props
We won't open a debate over which is better as in our opinion both have their use case and there is no reason to just use one of the two options.
So you already saw before that you can use the withNamespaces to decorate your component to pass the t function down.
The same works with a render prop:
import React from 'react';
​
// the render prop
import { NamespacesConsumer } from 'react-i18next';
​
export default function Page2 () {
  (
    <NamespacesConsumer ns={['page2', 'common']}>
      {
        (t, { i18n, ready }) => (
          ready ?
            <h1>{t('Welcome on page2')}</h1> :
            <p>{t('common:loading')}</p>
        )
      }
    </NamespacesConsumer>
  )
}
5) Translate JSX nodes as one string
Let's translate this:
Translating content with formatting or a link is a pain.
For this your JSX might look like:
<p>
  Translating content with 
  <strong>formatting</strong> 
  or a 
  <a href="#">link</a> 
  is a pain.
</p>
So naiv approach using the t function would result in
<p>
  { t('Translating content with ') }
  <strong>t('formatting')</strong> 
  { t(' or a ') }
  <a href="#">t('link')</a> 
  { t(' is a pain.') }
</p>
So you end up with 5 keys in your translation file and your translator has no idea about how this 5 keys relate to each other. Further what happens in other languages where the order needs to be changed?!?
So you won't have luck with this approach.
Using the Trans component
The Trans component enables you to keep this as one sentence by replacing the JSX nodes with indexed pseudo tags.
import { Trans } from 'react-i18next';
​
// ...
​
<p>
  <Trans>
    Translating content with 
    <strong>formatting</strong> 
    or a 
    <a href="#">link</a> 
    is a pain.
  </Trans>
</p>
Resulting in JSON:
{
  "Translating content with <1>formatting</1> or a <3>link</3> is a pain.": 
"Translating content with <1>formatting</1> or a <3>link</3> is a pain."
}
The <1>, <3> pseudo tags are based on the index of appearance in nodes.children:
<Trans>
  Translating content with     // index 0
  <strong>formatting</strong>  // index 1
  or a                         // index 2
  <a href="#">link</a>         // index 3
  is a pain.                   // index 4
</Trans>
The Trans component also supports interpolation and plurals just read the full documentation of that component.
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