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Choose between a header and a locale
An Accept-Language value tells a server which languages the client prefers. A value such as fr-FR,fr;q=0.9 requests French as the primary preference and generic French as a fallback. The header alone does not necessarily change what JavaScript reports or how the browser formats dates and numbers.
Use this decision rule:
- Only the HTTP value matters: configure extra HTTP headers in Puppeteer, Playwright, or CDP.
- The page should behave like a user in a locale: use Playwright’s
localeoption, or CDP’s browser-language emulation. - You need both: set the locale and verify the actual request; add an explicit header only when the exact wire value is required.
These settings affect requests made after they are configured. Apply them before goto() or the first navigation you intend to test.
Playwright: emulate a locale or set only the header
Full locale emulation
Playwright’s browser-context locale option changes three observable areas: navigator.language, the Accept-Language request header, and number/date formatting rules. This is the appropriate choice for tests that represent a user in a particular region.
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import { chromium } from 'playwright';
const browser = await chromium.launch({ headless: true });
const context = await browser.newContext({ locale: 'fr-FR' });
const page = await context.newPage();
await page.goto('https://example.com', { waitUntil: 'domcontentloaded' });
console.log(await page.evaluate(() => navigator.language));
await browser.close();A locale such as en-GB or de-DE represents a language and regional variant. Let Playwright construct the corresponding browser signals when those signals are what your application uses.
Exact header injection
When the test must send a particular string, set extraHTTPHeaders on the context:
import { chromium } from 'playwright';
const browser = await chromium.launch({ headless: true });
const context = await browser.newContext({
extraHTTPHeaders: {
'Accept-Language': 'fr-FR,fr;q=0.9'
}
});
const page = await context.newPage();
await page.goto('https://example.com');
await browser.close();Context-level extra headers apply to requests initiated by pages in that context. If you also configure page-specific headers, the page-level value takes precedence for a header overridden there. Use this narrowly when a protocol- or application-specific string matters more than complete locale behavior.
Playwright validation
const observed = await page.evaluate(() => ({
language: navigator.language,
languages: navigator.languages,
formatted: new Intl.NumberFormat().format(1234567.89),
date: new Intl.DateTimeFormat().format(new Date('2025-01-15T00:00:00Z'))
}));
console.log(observed);To verify the wire value, have the test server record request headers or use its request-inspection endpoint. Checking only navigator.language can miss a header-only configuration; checking only the server can miss a locale mismatch in client-side rendering.
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Puppeteer exposes page.setExtraHTTPHeaders(). The headers are sent with every request initiated by that page after the method is called.
import puppeteer from 'puppeteer';
const browser = await puppeteer.launch({ headless: true });
const page = await browser.newPage();
await page.setExtraHTTPHeaders({
'Accept-Language': 'fr-FR,fr;q=0.9'
});
await page.goto('https://example.com', { waitUntil: 'domcontentloaded' });
console.log(await page.evaluate(() => navigator.language));
await browser.close();Call the method before goto(), because the initial document request occurs during navigation. The setting also applies to subsequent requests initiated by that page, including resource requests and navigations, subject to browser and protocol behavior.
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Header-name and ordering caveats
Puppeteer’s documented API lowercases header names and does not guarantee outgoing header order. HTTP header names are case-insensitive, so an assertion should compare names without relying on capitalization. Do not write a test that expects Accept-Language to appear before or after another header.
Puppeteer is a framework interface over Chrome automation (and also supports Firefox); this API is not a special command-line switch that applies to every headless launcher.
Chrome DevTools Protocol: control the Network domain
CDP provides two related methods. Network.setUserAgentOverride accepts an acceptLanguage parameter described as the browser language to emulate. That method also requires the userAgent value, so provide the existing or intended user-agent string as well. Network.setExtraHTTPHeaders injects a header map for page requests without asserting broader locale behavior.
Using acceptLanguage
const client = await page.target().createCDPSession();
await client.send('Network.enable');
await client.send('Network.setUserAgentOverride', {
userAgent: await page.evaluate(() => navigator.userAgent),
acceptLanguage: 'fr-FR,fr;q=0.9'
});
await page.goto('https://example.com');Use the CDP client/session creation method provided by your automation library. The important ordering is enabling the Network domain, sending the override before navigation, and supplying the required userAgent field together with acceptLanguage.
Using CDP extra headers
const client = await page.target().createCDPSession();
await client.send('Network.enable');
await client.send('Network.setExtraHTTPHeaders', {
headers: {
'Accept-Language': 'fr-FR,fr;q=0.9'
}
});
await page.goto('https://example.com');This is the CDP equivalent of direct header injection. Choose it when the exact request header is the requirement and you do not need JavaScript-visible locale or formatting changes.
Protocol-version considerations
CDP evolves with Chrome. Check the protocol documentation that matches the Chrome build installed in CI or production, and keep the automation library and browser versions compatible. A method or parameter accepted by one Chrome version should not be assumed to behave identically in another without verification.
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Accept-Language syntax and useful test cases
The value is a comma-separated preference list. A quality value (q) expresses relative preference; omit it for the first choice or use a value between zero and one for fallbacks.
| Value | Meaning | Typical use |
|---|---|---|
en-US |
U.S. English only | Test a single regional locale. |
fr-FR,fr;q=0.9 |
French as used in France, then generic French | Test regional fallback. |
de-DE,de;q=0.9,en;q=0.8 |
German regional preference, German fallback, then English | Exercise server negotiation with multiple choices. |
* |
Any language is acceptable | Test wildcard handling; do not confuse it with a concrete locale. |
Include a regional tag only when the application distinguishes regions. Otherwise, a language-only value such as fr may be sufficient. Test the fallback order your server actually implements rather than assuming every framework resolves it identically.
How to verify the result
Inspect the request at the server
The definitive check for a header test is the request received by the server. Log the Accept-Language field for the navigation request and, when relevant, for an API or subresource request. This catches configuration applied to the wrong page, context, or navigation.
Inspect browser-visible values
For locale emulation, inspect navigator.language and navigator.languages, then render a number and date with Intl.NumberFormat and Intl.DateTimeFormat. A correct header with an unchanged navigator.language is expected when you deliberately chose header-only injection.
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Check redirects and new contexts
Follow the complete redirect chain in server logs. A new browser context or page created after configuration does not inherit page-level Puppeteer settings automatically. In Playwright, ensure the page belongs to the context where the option was set. Repeat the check after redirects and client-side navigations.
Common failures and fixes
The server still sees the default language
- Set the header or locale before the first
goto(). - Confirm the request came from the configured page/context, not a separately created one.
- Inspect the final request after redirects, not only an initial client-side event.
- Check that a proxy, service worker, or upstream gateway is not replacing the header.
navigator.language did not change
You probably injected only a request header. Use Playwright locale or CDP’s acceptLanguage emulation when JavaScript-visible language is part of the behavior under test.
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Date and number formatting remain unchanged
Extra headers do not promise formatting changes. Configure the Playwright context’s locale and verify with the Intl APIs. Also make sure your assertion accounts for the browser’s locale data and time zone; language selection alone does not set a time zone.
The CDP call fails with a missing parameter
Network.setUserAgentOverride requires a userAgent value in addition to acceptLanguage. Read the protocol definition for the Chrome version in use and send all required fields.
An assertion fails because of capitalization or order
In Puppeteer, header names are lowercased and header order is not guaranteed. Compare the case-insensitive name and value, not the serialized order.
Only the first request has the desired value
Check whether the setting was attached to a page that was later replaced, or whether another context-level or page-level header override took precedence. Configure the stable context/page used for all navigations and inspect a later request as well as the document request.
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Header configuration itself is lightweight; the cost usually comes from the navigation and the page work you are testing. Reuse a browser process when appropriate, but isolate tests that require different locales in separate contexts. In Playwright, contexts provide a clean boundary for locale and extra-header settings. In Puppeteer, create and configure each page deliberately so a prior test’s headers do not leak into the next one.
Prefer deterministic server fixtures for assertions. Real sites can select content using cookies, geolocation, account settings, or cached responses in addition to Accept-Language. If a test is intended to prove language negotiation, record the received header and response language independently. If it is intended to prove the complete user experience, assert the DOM text, navigator.language, and formatting output together.
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Pin or regularly validate the Chrome and automation-library versions used by CI. CDP methods and framework APIs can evolve, and a successful local run does not prove that a different installed Chrome build exposes the same protocol surface.
Or skip the browser setup
If your goal is a clean screenshot rather than an interactive browser test, ScreenshotNeo accepts a URL and returns a PNG, JPEG, WebP, or PDF. You can pass the language header through its custom-header option while avoiding local Chrome setup. See the ScreenshotNeo documentation for the current parameter names and complete option list.
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -d 'headers[Accept-Language]=fr-FR,fr;q=0.9' -o shot.webpScreenshotNeo can accept cookie and consent banners before capture and remove more than 60 known consent platforms, newsletter popups, and chat widgets; each cleanup step can be turned off. Bot checks or CAPTCHAs, blank pages, timeouts, failed loads, and cache hits are not billed, and the response identifies the result with X-Page-Verdict and X-Billed headers. Its MCP server provides take_screenshot, get_page_info, and capture_pdf tools for Claude, Cursor, and other MCP clients.
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Frequently Asked Questions
Should I set locale or Accept-Language in Playwright?
Use locale for complete locale behavior, including navigator.language and date/number formatting. Use extraHTTPHeaders when only the exact request header matters.
Does changing Accept-Language change the browser’s time zone?
No. Language selection and time-zone emulation are separate settings; test or configure the time zone independently when it affects output.
Can I rely on a Chrome launch flag for this header?
The documented approaches here are CDP and framework APIs. Do not assume one launch argument works consistently across launchers and Chrome versions without checking that exact combination.
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