Implementing Localization in a Next.js App

I just finished implementing localization for a production Next.js app to support English, Spanish, and Portuguese and here's what I learned along the way.

In this article you will learn how I set up a robust internationalization (i18n) system using i18next and react-i18next.

The Tech Stack 🛠️

We went with a battle-tested stack for this:

• i18next: The core framework. It's been around forever and just works.
• react-i18next: The React bindings (because hooks are life).
• i18next-browser-languagedetector: Automatically detects the user's language preference from their browser or localStorage.

Supported Languages

• English (en) - Default/Fallback
• Spanish (es)
• Portuguese (pt)

Configuration ⚙️

The heart of our i18n setup lives in src/i18n/index.ts. This file initializes i18next, loads our translation resources, and configures detection.

Here's what the configuration looks like:

// src/i18n/index.ts
import i18n from 'i18next';
import { initReactI18next } from 'react-i18next';
import LanguageDetector from 'i18next-browser-languagedetector';

import en from './locales/en.json';
import es from './locales/es.json';
import pt from './locales/pt.json';

i18n
  .use(LanguageDetector)
  .use(initReactI18next)
  .init({
    resources: {
      en: { translation: en },
      es: { translation: es },
      pt: { translation: pt },
    },
    fallbackLng: 'en',
    interpolation: {
      escapeValue: false, // React already safe from XSS
    },
    detection: {
      order: ['localStorage', 'navigator'],
      caches: ['localStorage'],
    },
  });

export default i18n;
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Our translation strings are stored in simple JSON files:

src/i18n/locales/
├── en.json
├── es.json
└── pt.json
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Usage in Components 🧩

Using the translations is super straightforward thanks to the useTranslation hook.

import { useTranslation } from 'react-i18next';

function WelcomeHeader() {
  const { t } = useTranslation();

  return (
    <header className="p-4 bg-primary text-white">
      <h1>{t('welcomeMessage')}</h1>
      <p>{t('subtitle')}</p>
    </header>
  );
}
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The Language Switcher 🔀

We built a custom LanguageSwitcher component to let users swap languages easily. It features a dropdown menu, persists the selection, and even has some nice animations! ✨

Here is the full component code:

import { useState, useRef, useEffect, type JSX } from 'react';
import { useTranslation } from 'react-i18next';
import { Check, ChevronDown } from 'lucide-react';

const LANGUAGES = [
  { code: 'en', label: 'EN' },
  { code: 'es', label: 'ES' },
  { code: 'pt', label: 'PT' },
] as const;

export function LanguageSwitcher(): JSX.Element {
  const { i18n } = useTranslation();
  const [isOpen, setIsOpen] = useState(false);
  const menuRef = useRef<HTMLDivElement>(null);

  useEffect(() => {
    if (!isOpen) return;

    const handleClickOutside = (event: MouseEvent) => {
      if (
        menuRef.current &&
        event.target instanceof Node &&
        !menuRef.current.contains(event.target)
      ) {
        setIsOpen(false);
      }
    };

    document.addEventListener('mousedown', handleClickOutside);
    return () => document.removeEventListener('mousedown', handleClickOutside);
  }, [isOpen]);

  const currentLanguage =
    LANGUAGES.find((l) => l.code === i18n.language) || LANGUAGES[0];

  return (
    <div className="relative" ref={menuRef}>
      <button
        type="button"
        onClick={() => setIsOpen((prev) => !prev)}
        className="flex items-center gap-2 rounded-md border border-border bg-background px-3 py-2 text-sm font-medium text-foreground hover:bg-accent focus:outline-none"
        aria-haspopup="listbox"
        aria-expanded={isOpen}
      >
        <span>{currentLanguage.label}</span>
        <ChevronDown
          className={`h-4 w-4 transition-transform duration-200 ${
            isOpen ? 'rotate-180' : ''
          }`}
        />
      </button>

      {isOpen && (
        <div className="absolute right-0 z-50 mt-2 w-32 overflow-hidden rounded-md border border-border bg-popover shadow-md animate-in fade-in-0 zoom-in-95">
          <div className="p-1">
            {LANGUAGES.map(({ code, label }) => {
              const isSelected = i18n.language === code;
              return (
                <button
                  key={code}
                  onClick={() => {
                    i18n.changeLanguage(code);
                    setIsOpen(false);
                  }}
                  className={`flex w-full items-center justify-between rounded-sm px-2 py-1.5 text-sm text-left outline-none transition-colors hover:bg-accent hover:text-accent-foreground ${
                    isSelected ? 'bg-accent text-accent-foreground' : ''
                  }`}
                  role="option"
                  aria-selected={isSelected}
                >
                  <span>{label}</span>
                  {isSelected && <Check className="h-4 w-4" />}
                </button>
              );
            })}
          </div>
        </div>
      )}
    </div>
  );
}
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API Integration & Headers 🌐

It's not just the UI that needs to be localized—the backend needs to know what language the user prefers too!

We handled this with a simple Axios interceptor that attaches the Accept-Language header to every outgoing request.

// In src/services/api.ts
api.interceptors.request.use((config) => {
  // Pass the current language to the backend
  config.headers['Accept-Language'] = i18n.language;
  return config;
});
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Now our API responses (like validation errors or dynamic data) can respect the user's choice automatically. 😎

Validation Messages (Zod) ✅

Speaking of validation, we're using Zod for our schemas. To localize error messages, we created a utility function syncZodLocale.

It maps the current i18next language to Zod's built-in locales (z.locales.en, z.locales.es, etc.). We trigger this synchronization whenever the app loads or the language changes. No more English messages coming from the backend for Spanish users!

Testing 🧪

Testing localized components can be tricky because localStorage persists between tests, which can lead to flaky results.

To solve this, we use a helper function createTestI18n that creates an isolated instance of i18next for each test.

// src/test/i18nTestHelper.ts
export const createTestI18n = (language = 'en') => {
  const i18n = i18next.createInstance();
  i18n.init({
    lng: language,
    fallbackLng: 'en',
    resources: {
      en: { translation: en },
      es: { translation: es },
      // ...
    },
  });
  return i18n;
};
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Usage in Tests:

import { render } from '@testing-library/react';
import { I18nextProvider } from 'react-i18next';
import { createTestI18n } from '../../../test/i18nTestHelper';

test('renders in Spanish', () => {
  const i18n = createTestI18n('es'); // Start test in Spanish
  render(
    <I18nextProvider i18n={i18n}>
      <LanguageSwitcher />
    </I18nextProvider>,
  );
  // Assert Spanish content...
});
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Backend Bonus: FastAPI & Babel 🐍

Wait, there's more! For the backend, we used the fastapi-babel package, and honestly, it's pretty slick.

In the frontend, we're used to using dot-notation keys like t('home.welcomeMessage'). But in the backend, we used natural language keys.

Instead of home.welcomeMessage, the key is the English string:

_("Welcome to the API")
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This makes the backend code so much more readable! You don't have to jump to a translation file to know what a message says—it's right there in the code. 🤯

Conclusion

Implementing localization might seem daunting at first, but with the right tools, it becomes a breeze. 🌬️ The combination of i18next for the frontend and fastapi-babel for the backend gave us a seamless, end-to-end multi-language experience.

Now go out there and make your apps global! 🌎

Happy coding! 🚀