Locale detection strategies in NextJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get the user's preferred language from the browser in Next.js.

NextJS

const userLocale = [1] || 'en';

Drag options to blanks, or click blank then click option'

Anavigator.language

Bnavigator.languages

Cnavigator

Dnavigator.userLanguage

Attempts:

3 left

💡 Hint

Common Mistakes

Using navigator.languages returns an array, not a string.

Using navigator.userLanguage is only supported in some browsers.

The navigator.language property returns the browser's preferred language as a string, such as 'en-US'.

2fill in blank

medium

Complete the code to detect locale from the URL pathname in Next.js.

NextJS

const locale = router.[1].split('/')[1] || 'en';

Drag options to blanks, or click blank then click option'

Alocale

BasPath

Cquery

Dpathname

Attempts:

3 left

💡 Hint

Common Mistakes

Using router.pathname which may not reflect the actual URL.

Using router.query which is an object, not a string.

router.asPath gives the actual URL path including locale prefix, which we split to get the locale.

3fill in blank

hard

Fix the error in this Next.js middleware code to correctly detect locale from the request headers.

NextJS

const locale = req.headers.get('[1]')?.split(',')[0] || 'en';

Drag options to blanks, or click blank then click option'

Auser-language

Bcontent-language

Caccept-language

Dlocale

Attempts:

3 left

💡 Hint

Common Mistakes

Using content-language which is a response header.

Using user-language which is not a standard header.

The accept-language header contains the user's preferred languages in order.

4fill in blank

hard

Fill both blanks to create a Next.js middleware that redirects to the locale path if missing.

NextJS

if (!req.nextUrl.pathname.startsWith('/[1]')) {
  return NextResponse.redirect(new URL('/[2]' + req.nextUrl.pathname, req.url));
}

Drag options to blanks, or click blank then click option'

Aen

Bfr

Ces

Dde

Attempts:

3 left

💡 Hint

Common Mistakes

Using different locale codes in the two blanks causing inconsistent redirects.

Forgetting the leading slash in the path.

This middleware checks if the path starts with '/en' and redirects to add '/en' if missing.

5fill in blank

hard

Fill all three blanks to create a Next.js server component that shows the detected locale from headers.

NextJS

export default async function LocaleDisplay() {
  const locale = headers().get('[1]')?.split(',')[0] || '[2]';
  return <p>Your locale is: [3]</p>;
}

Drag options to blanks, or click blank then click option'

Aaccept-language

Ben

C{locale}

Dcontent-language

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'content-language' header which is not sent by browsers.

Not wrapping the locale variable in curly braces in JSX.

The component reads the 'accept-language' header, defaults to 'en', and displays the locale inside a paragraph.

Practice

1. What is the main purpose of locale detection in a Next.js application?

easy

A. To change the website layout based on screen size

B. To improve server response time by caching pages

C. To track user clicks for analytics

D. To automatically show the website in the user's preferred language

Solution

1. Step 1: Understand locale detection purpose

   Locale detection is used to identify the user's language preference automatically.

2. Step 2: Match purpose to options

   Showing the website in the user's preferred language matches the purpose of locale detection.

3. Final Answer:

   To automatically show the website in the user's preferred language -> Option D

4. Quick Check:

   Locale detection = show preferred language [OK]

Hint: Locale detection means showing language automatically [OK]

Common Mistakes:

• Confusing locale detection with performance optimization
• Thinking it changes layout instead of language
• Mixing locale detection with analytics tracking

2. Which Next.js feature is commonly used to configure supported locales for automatic detection?

easy

A. next.config.js i18n field

B. pages/api/locale.js file

C. Custom _document.js file

D. Static CSS files

Solution

1. Step 1: Identify where locale config is set

   Next.js uses the i18n field inside next.config.js to define supported locales.

2. Step 2: Check other options

   API files, _document.js, and CSS files do not configure locales.

3. Final Answer:

   next.config.js i18n field -> Option A

4. Quick Check:

   Locale config = next.config.js i18n [OK]

Hint: Locales go in next.config.js under i18n [OK]

Common Mistakes:

• Trying to configure locales in API routes
• Using _document.js for locale settings
• Assuming CSS files control language

3. Given this middleware snippet in Next.js, what happens when a user with 'fr' browser language visits the site?

export function middleware(request) {
  const locale = request.headers.get('accept-language')?.split(',')[0].split('-')[0];
  if (locale === 'fr') {
    return Response.redirect(new URL('/fr', request.url));
  }
  return Response.next();
}

medium

A. User is redirected to the homepage

B. User is redirected to the /fr page

C. User gets a 404 error

D. User stays on the current page without redirect

Solution

1. Step 1: Analyze locale extraction from headers

   The code extracts the first language code from the 'accept-language' header, e.g., 'fr'.

2. Step 2: Check redirect condition

   If locale is 'fr', the middleware redirects to '/fr' URL.

3. Final Answer:

   User is redirected to the /fr page -> Option B

4. Quick Check:

   Locale 'fr' triggers redirect to /fr [OK]

Hint: Check header locale then redirect if matches [OK]

Common Mistakes:

• Ignoring the redirect and assuming no change
• Confusing header parsing logic
• Assuming redirect to homepage always

4. Identify the error in this Next.js middleware code for locale detection:

export function middleware(request) {
  const locale = request.headers.get('accept-language').split(',')[0];
  if (locale === 'en') {
    return Response.redirect(new URL('/en', request.url));
  }
  return Response.next();
}

medium

A. Missing optional chaining on headers.get could cause runtime error

B. Redirect URL is incorrect, should be '/en-US'

C. Response.next() is not a valid method

D. Locale comparison should use locale.includes('en')

Solution

1. Step 1: Check headers.get usage

   headers.get('accept-language') can return null if header missing, so calling split() without check causes error.

2. Step 2: Identify fix

   Using optional chaining like headers.get('accept-language')?.split(',')[0] prevents runtime error.

3. Final Answer:

   Missing optional chaining on headers.get could cause runtime error -> Option A

4. Quick Check:

   Null header needs optional chaining [OK]

Hint: Always check headers.get may return null [OK]

Common Mistakes:

• Assuming headers.get always returns string
• Incorrect redirect URL format
• Using invalid Response methods

5. You want to detect user locale in Next.js by checking cookie first, then header, and fallback to default 'en'. Which code snippet correctly implements this logic in middleware?

hard

A. const locale = 'en' || request.cookies.get('NEXT_LOCALE') || request.headers.get('accept-language');

B. const locale = request.headers.get('accept-language')?.split(',')[0].split('-')[0] || request.cookies.get('NEXT_LOCALE') || 'en';

C. const locale = request.cookies.get('NEXT_LOCALE') || request.headers.get('accept-language')?.split(',')[0].split('-')[0] || 'en';

D. const locale = request.cookies.get('NEXT_LOCALE') && request.headers.get('accept-language') || 'en';

Solution

1. Step 1: Understand priority order

   We want to check cookie first, then header, then default 'en'.

2. Step 2: Analyze each option

   const locale = request.cookies.get('NEXT_LOCALE') || request.headers.get('accept-language')?.split(',')[0].split('-')[0] || 'en'; checks cookie first, then header with safe optional chaining, then 'en'. Others have wrong order or logic.

3. Final Answer:

   const locale = request.cookies.get('NEXT_LOCALE') || request.headers.get('accept-language')?.split(',')[0].split('-')[0] || 'en'; -> Option C

4. Quick Check:

   Cookie first, then header, then default [OK]

Hint: Use || in order: cookie, header, default [OK]

Common Mistakes:

• Checking header before cookie
• Using && instead of || causing wrong logic
• Missing optional chaining on header